Introduction: Why Form 64-8 Matters More When You File in Two Countries
Form 64-8 is the HMRC document that authorises a paid tax agent to deal with the UK tax authority on your behalf, and it is only half of the authority an American in Britain needs. HMRC will not discuss your affairs with anyone until that authority sits on its system. Likewise, the IRS will not speak to your representative without its own, entirely separate power of attorney. Consequently, a dual filer needs two sets of authorisations that follow different rules, last for different periods and cover different things. When one of them is missing, returns stall, letters go unanswered and penalties start.
Form 64-8 in One Paragraph
Form 64-8 is a three-page HMRC form titled "Authorising your agent". You complete it, tick the taxes you want the agent to handle, sign it and post it to HMRC. It can cover Self Assessment, PAYE for individuals, Corporation Tax, PAYE for employers, the Construction Industry Scheme, VAT and, since December 2025, Making Tax Digital for Income Tax. However, it does not reach every HMRC service, it gives no authority at all before the IRS, and it does not expire on its own. Therefore old authorisations linger long after you change advisers.
Why Dual Filers Get This Wrong
Most guides to Form 64-8 assume a purely British taxpayer with one accountant. Wealthy Americans in Britain rarely fit that picture. You may use a UK accountant for Self Assessment, a US preparer for Form 1040 and a third firm for your company. Moreover, your US preparer may sit in New York and have no standing with HMRC whatsoever. At TaxYork we prepare both the US and UK returns, so we hold both sets of authority for the same client. This guide explains how Form 64-8 works in 2026, where it stops, and how it fits beside IRS Form 2848.
What Form 64-8 Covers in 2026
HMRC updated its guidance on Form 64-8 several times during 2026, most recently on 6 October 2026. As a result, much of what you read online is out of date. The current position appears in HMRC's Form 64-8 publication page, and four points matter most.
The Taxes You Can Tick
Form 64-8 works tax by tick box. For an individual, the key boxes are Self Assessment and PAYE for individuals. A Self Assessment authority lets your agent submit your return, claim reliefs, discuss the return with HMRC, change your details, give bank details for repayments and view your calculations. Meanwhile, the individual PAYE authority opens your employment history, taxable benefits and pension records. For a company owner, the form also covers Corporation Tax and PAYE for employers. Furthermore, HMRC added Making Tax Digital for Income Tax to the form's permissions on 12 December 2025, which matters for landlords and the self-employed now inside that regime.
One Agent Per Form, and New Replaces Old
Each Form 64-8 names a single agent. If you use one firm for your personal return and another for your company, you complete one form for each. Importantly, a new authority for the same tax replaces the existing one. HMRC states that any new authority received replaces any existing authority. Therefore a careless form can silently remove an agent you still need. Accordingly, list which firm handles which tax before you sign anything.
You Must Be Registered First
You cannot authorise an agent for Self Assessment until you are registered for it. Newly arrived Americans often miss this step. You first register for Self Assessment and receive a ten-digit Unique Taxpayer Reference. Only then can Form 64-8 attach an agent to your record. Consequently, an executive who lands in London in the spring should register well before the first January deadline, because registration and authorisation each take weeks.
You Stay Responsible
Authorising an agent does not transfer your legal duties. HMRC's guidance on authorising an agent to deal with your tax affairs says plainly that you remain legally responsible for your own tax. Additionally, you must check your return and confirm it is correct before your agent submits it. The IRS takes the same view of a US return. In both countries, the signature and the consequences stay with you.
What Form 64-8 Does Not Cover
The limits of Form 64-8 cause more trouble than its contents. Several HMRC services now sit outside it, and three of them matter to high-net-worth Americans.
Services That Need a Digital Handshake
Some newer HMRC services ignore the paper form entirely. Instead, your agent sends you a link from their agent services account, and you approve the request by signing in to your own HMRC account. HMRC calls this the digital handshake. The Capital Gains Tax on UK Property account uses it, so a Form 64-8 will not let your agent file the 60-day return when you sell a London flat. Similarly, HMRC's online services handbook for agents confirms that an existing 64-8 gives no access to the Income Record Viewer, which shows your pay, tax codes and employment history. We cover the client side of these accounts in our guide to the HMRC personal tax account for Americans.
Limited Authority and Compliance Checks
HMRC no longer accepts Form 64-8 for a limited, one-off authority. If you want a firm to handle a single issue, you now write a signed letter of authority that names the agent and gives your tax reference. Furthermore, since July 2026 HMRC's guidance directs you to a separate form, COMP1a, where you want an agent to help with a compliance check. Therefore, if HMRC opens an enquiry into your return, do not assume your standing authority is enough. Confirm the position with your agent on the day the letter arrives.
Friends, Family and Shared Logins
Form 64-8 is for paid agents only. A spouse or relative who helps without payment uses the separate route for friends and family. More importantly, never hand your Government Gateway login to anyone, including an accountant. An agent with proper authority works through their own agent account. A shared login leaves no record of who did what, and HMRC tells taxpayers not to share sign-in details.
How to Authorise Your Agent: Three Routes Compared
In practice, your agent chooses the route. Nevertheless, you should understand all three, because the right choice depends on where your post goes.
Online Agent Authorisation and the Posted Code
Under the online agent authorisation service, your agent requests access and HMRC posts a letter containing a code to you within seven working days. You then pass the code to the agent, usually within 30 days of the letter's date. After that the code expires and the process starts again. Notably, each tax needs its own letter and its own code. This route fails regularly for internationally mobile clients. If HMRC holds an old address, or an address in the United States, the letter may arrive after the code has died. Consequently, check the address HMRC holds before your agent presses the button.
The Paper Form 64-8
The paper Form 64-8 needs no code. You download it, complete it on screen or by hand, sign and date it, and post it to the address printed on the form. HMRC asks you to write only inside the boxes and to leave out covering letters unless they are essential. In addition, your agent's codes must be exact, including the six-character Self Assessment agent code. HMRC publishes no fixed turnaround, and in our experience you should allow four to six weeks. Therefore the paper route suits a client whose post is unreliable, but it does not suit a client with a deadline next week.
The Digital Handshake
The digital handshake is the quickest route where it applies. You receive a link, sign in and approve the request, and the authority usually takes effect straight away. However, it works only if you can sign in to your own HMRC account. Many Americans struggle with identity checks that expect a UK passport or UK credit history. Accordingly, set up and test your own HMRC sign-in early, well before you need an agent to act.
The 2026 Registration Rules and Your US-Based Preparer
A major change took effect this year, and it affects every American who relies on a preparer outside Britain. Since 18 May 2026, anyone who is paid to interact with HMRC about another person's tax affairs must register with HMRC through an agent services account.
Why a US Preparer Cannot Simply Be Named
Form 64-8 asks for the agent's HMRC codes. A firm with no HMRC registration has none to give. Moreover, HMRC's manual on overseas tax advisers confirms that overseas advisers who interact with HMRC on behalf of clients must also register, unless a specific exception applies. Therefore your New York CPA cannot telephone HMRC about your UK refund on the strength of a signed letter and goodwill. Either that firm registers, or a UK-registered agent acts.
The Phased Deadlines
Registration opened in windows. Advisers with no existing HMRC agent, Self Assessment or Corporation Tax account had from 18 May to 18 August 2026. Those with such an account but no agent services account have until 18 November 2026. Payroll-only advisers follow, from 18 November 2026 to 18 February 2027. The professional bodies have published detailed commentary on the registration guidance and answers to common registration questions. Additionally, one institute has noted that HMRC's agent registration guidance still lacks key details.
What to Ask Your Preparer Now
Ask one direct question: is the firm registered with HMRC, and under which agent codes? If the answer is unclear, your UK filing position depends on someone HMRC may refuse to hear. Similarly, ask who holds your Form 64-8 today. Many clients discover that a firm they left years ago still appears on their record. Our guide to changing expat tax accountant sets out the full handover sequence.
Form 64-8 and IRS Form 2848: Two Systems Side by Side
Form 64-8 has an American counterpart, and the two differ in almost every respect. Understanding the contrast prevents the most common dual-filer mistake, which is to assume that one authority covers both countries.
What Form 2848 Does
Form 2848 is the IRS power of attorney. It authorises a named individual to represent you before the IRS and to receive your confidential tax information. Crucially, the representative must be eligible to practise before the IRS, which generally means an attorney, a certified public accountant or an enrolled agent. A UK chartered accountant without one of those credentials cannot represent you. We compare the credentials in our guide to enrolled agents and CPAs.
Specific Years Versus an Open-Ended Authority
Form 64-8 authorises a tax, with no end date. In contrast, Form 2848 authorises specific matters, specific form numbers and specific years. According to the Form 2848 instructions, the IRS will not record any future year that falls more than three years after the end of the year in which it receives the form. Therefore a US authority needs renewal, while a UK authority needs deliberate removal. Furthermore, a new Form 2848 generally revokes an earlier one for the same matter unless you tick the box to retain it. On a joint return, each spouse signs a separate form.
Sending Form 2848 From Britain
Taxpayers abroad send Form 2848 to the IRS International CAF Team in Philadelphia by post or fax. Alternatively, your representative can submit Forms 2848 and 8821 online. Importantly, a posted or faxed form needs a handwritten signature, while an electronic signature works only for online submission. The lighter Form 8821 lets a firm receive your information without representing you, which is often enough to obtain transcripts. Meanwhile, FBAR filing needs its own signed authority, as the IRS guidance on the FBAR explains. Our guide to the IRS online account for Americans in Britain covers the digital options.
Why Both Authorities Must Sit With Coordinated Preparers
The two returns feed each other. Your US foreign tax credit depends on UK tax actually paid, which only HMRC's record proves. Conversely, your UK return may claim credit for US tax on American income. If your UK agent cannot see HMRC's calculation, or your US representative cannot pull an IRS transcript, each return rests on guesswork. Consequently, the authorities are not paperwork for its own sake. They are the means by which both returns agree.
Missed Returns, Penalties and Removing an Old Agent
Problems with Form 64-8 rarely surface as authorisation problems. Instead, they surface as a missed UK tax return, an unanswered letter or a penalty notice.
How a Stale Authority Leads to Missed UK Tax Returns
A stale authority leaves a firm you have left with access to your record, while HMRC keeps writing to whatever address it holds. Moreover, your new preparer cannot see your record until their own authority is processed. During that gap, filing deadlines pass. The Self Assessment penalty rules then apply automatically: £100 at once, £10 a day for up to 90 days after three months, and further charges at six and twelve months.
Reliance on an Agent Is Rarely an Excuse
Many taxpayers assume that an agent's failure excuses the penalty. It usually does not. Under Schedule 55 to the Finance Act 2009, reliance on another person is not a reasonable excuse unless you took reasonable care to avoid the failure. Therefore you need evidence that you chased, checked and followed up. Where the error was careless but not deliberate, other reliefs can sometimes help, as our guide to HMRC suspended penalties explains.
Removing an Agent Properly
You can remove an agent online through your personal or business tax account, or in writing. HMRC's guidance on how to change or remove your tax agent's authorisation lists the steps for each tax. Notably, removing an agent from Self Assessment does not remove them from Making Tax Digital for Income Tax, so you must do both. Additionally, HMRC warns that it may ask you to reauthorise an agent in future to comply with data protection law. Accordingly, review your authorities once a year, on both sides of the Atlantic.
Case Study: A Private Equity Partner and a £2,590 Penalty
The following illustration uses realistic figures. We have changed the details to protect confidentiality.
The Starting Position
Claire is a US citizen and a partner at a private equity firm in London. She moved from Manhattan in 2023. In January 2026 she left her UK accountant and asked her long-standing New York preparer to "handle everything". That firm prepared her Form 1040 competently. However, it held no HMRC registration and no agent codes. Furthermore, the old UK firm remained on HMRC's record under a Form 64-8 signed in 2023.
What Went Wrong
Her 2024/25 Self Assessment return fell due on 31 January 2026, with £38,400 of tax to pay. The New York firm tried the online route in March. HMRC posted the code to her former Manhattan address, and it expired unseen. Meanwhile, HMRC's reminders went to a London flat she had already left. Nobody filed. Claire came to us in late June, and we filed on 26 June, almost five months late. She faced the £100 fixed penalty and £570 of daily penalties for the 57 days from 1 May. In addition, a 5% late payment penalty of £1,920 had arisen on the unpaid tax. The total was £2,590, plus interest, and none of it is creditable or deductible on her US return.
What We Did
Firstly, we updated her address with HMRC and submitted a fresh Form 64-8 for Self Assessment and individual PAYE, which replaced the old firm's authority. Secondly, we filed the return and arranged payment within days, which stopped the six-month penalties from arising. Thirdly, Claire signed a Form 2848 covering her income tax matters for 2022 to 2025, so that we could obtain her IRS transcripts. Those showed that her 2025 foreign tax credit claim had assumed a UK payment she never made. Consequently, we corrected the US position before the IRS raised it. Finally, she removed the old agent from every tax through her personal tax account.
The Lesson
Claire had two capable advisers and no valid UK authority. Her penalty came from the gap between them, not from any error in the figures. Therefore the first task in any dual-filing engagement is to confirm who holds your Form 64-8 and your US power of attorney, in which country, and until when.
How TaxYork Can Help
TaxYork provides comprehensive tax preparation and compliance for high-net-worth Americans in Britain, dual nationals and company owners. Because we prepare both returns, one coordinated team works under your Form 64-8 and your Form 2848. We prepare your Self Assessment return and your Form 1040 together, so the tax paid in one country matches the credit claimed in the other. Additionally, we prepare your FBAR and Form 8938 through our FBAR and FATCA reporting service. Where earlier years are missing, our IRS Streamlined Filing service and our cross-border planning team bring both records up to date.
Conclusion
Form 64-8 is a short form with long consequences. It gives your agent access to Self Assessment, PAYE, Corporation Tax and more, yet it leaves out the Capital Gains Tax on UK Property account, the Income Record Viewer, limited authority and compliance checks. Moreover, it has no effect before the IRS, where Form 2848 follows different rules on who may act and for how long. Since May 2026, any paid adviser who deals with HMRC must also be registered. Therefore check three things today: who is on your HMRC record, whether your preparer is registered, and which years your US power of attorney covers. Above all, make sure the same coordinated team can see both tax records.
Contact Us
If you are unsure who holds your Form 64-8, or if a change of adviser has left a gap in your US or UK filings, speak to our team. You can book a consultation today, email hello@taxyork.com or call 020 3488 8606. We will review your authorities in both countries and prepare every return that is due.
Disclaimer
This article provides general information only and reflects the law and published guidance as at October 2026. It does not constitute tax, legal or financial advice, and you should not act on it without taking professional advice on your own circumstances. Tax rules change, and their application depends on individual facts. TaxYork accepts no liability for any loss arising from reliance on this article. Please contact us for a review of your own position.