Key takeaways

  • HMRC gives four main ways to authorise an agent: the digital handshake, Online Agent Authorisation, the client's business tax account and paper form 64-8.
  • VAT and Making Tax Digital for Income Tax use the digital handshake from your agent services account; form 64-8 does not cover them.
  • A digital handshake link expires after 21 days, and the client must use the sign-in details they used for that tax.
  • With Online Agent Authorisation HMRC posts a code to the client, which you must enter within 28 days or it expires.
  • Form 64-8 is postal only, must be signed and dated, and is for when no online route or business tax account option is available.
  • HMRC no longer accepts form 64-8 for limited authorisation; the client writes a letter instead.
  • Under MTD for Income Tax a client can have one main agent and any number of supporting agents, and only the main agent can submit the tax return.

To act for a client with HMRC you need their authorisation, and the route depends on the tax. VAT and Making Tax Digital for Income Tax use the digital handshake from your agent services account. Self Assessment, Corporation Tax and PAYE use Online Agent Authorisation, with paper form 64-8 as the fallback.

Which authorisation route applies to each tax?

HMRC’s authorisation guidance, updated on 7 October 2026, sets out the route for each tax. These are the ones most practices use:

TaxOnline routePaper route
Self AssessmentOnline Agent AuthorisationForm 64-8
MTD for Income TaxDigital handshakeNone
Corporation TaxOnline Agent AuthorisationForm 64-8
PAYEOnline Agent Authorisation or BTAForm 64-8
VATDigital handshakeNone
CISOnline Agent Authorisation or BTANone
Trust Registration ServiceDigital handshakeNone
CGT on UK propertyDigital handshakeNone

BTA means the client’s business tax account. Individual PAYE or National Insurance queries use form 64-8 only.

How does the digital handshake work?

The digital handshake is an authorisation link you send from your agent services account (ASA). HMRC’s handshake guidance gives the steps:

  1. Sign in to your agent services account.
  2. Select “Ask a client to authorise you”.
  3. Enter the client’s details, such as their National Insurance number, date of birth or tax reference.
  4. Copy the authorisation request link from the final page.
  5. Send the link to the client.

The link expires after 21 days. The client must sign in with the details they used to register for that tax, or create sign-in details from the link. Clients who cannot go online can call HMRC with the link and are passed to the Extra Support Team.

If you do not yet have an ASA, see our guide to the HMRC agent services account. Advisers with older agent accounts but no ASA must register as tax advisers by 18 November 2026.

How does Online Agent Authorisation work?

Online Agent Authorisation is the request you make from your HMRC online services for agents account. HMRC’s Self Assessment Manual at SAM101180 describes it as fully automated:

  1. You submit an authorisation request for the client.
  2. HMRC posts a code to the client.
  3. The client gives you the code only if they agree to you acting.
  4. You enter the code online to complete the authorisation.

The code expires if not entered within 28 days. When it is accepted, your details replace any existing agent on the client’s record. Allow time for the letter to reach the client before a filing deadline.

When do you need form 64-8?

Use form 64-8 only when the client cannot use an online route or their business tax account. HMRC’s page, updated 6 October 2026, sets these rules:

  • download the PDF, fill it in, then sign and date it
  • post it to the HMRC address on the form
  • use the right agent code for each tax, such as a 6-character Self Assessment code
  • do not add a covering letter unless it is essential
  • do not write outside the boxes or strike through unused ones

Form 64-8 does not cover everything. Use these alternatives instead:

  • Limited authorisation: the client writes a letter to HMRC at BX9 1AN, with their name, address, UTR, your details and their signature
  • Compliance checks: form COMP1a
  • Trusts: authorisation inside the Trust Registration Service
  • VAT and MTD: the digital handshake

Main and supporting agents under MTD

Making Tax Digital for Income Tax lets a client appoint more than one agent. HMRC’s guidance on choosing agents says:

  • If you prepare the tax return, then you need to be the main agent. There can be only one.
  • If you only keep records or send quarterly updates, then you can be a supporting agent. There is no limit on how many.
  • If you already act for the client in Self Assessment, then copy those authorisations into your ASA. HMRC says this makes you the main agent, but it does not sign the client up for MTD.

Supporting agents see only business and property income details and cannot submit the tax return. See how to sign up for MTD for Income Tax.

What advisers should do

  1. Authorise early. Postal codes and paper forms take time, so start at onboarding, not near 31 January.
  2. Track handshake links. Diary a reminder before the 21-day expiry.
  3. Check your MTD role. Confirm whether you are the main or a supporting agent for each client.
  4. Remove old authorisations. When a client leaves, delete them from both your ASA and online services account.

Authorisation follows your client due diligence, so build it into onboarding a new client. For problems, use the HMRC agent dedicated line, and see the HMRC standard for agents.

Frequently asked questions

Can I use form 64-8 for VAT?

No. VAT authorisation uses the digital handshake from your agent services account. Form 64-8 applies only to the older VAT (legacy) route.

The link expires after 21 days. If the client does not use it in time, you must send a new request.

How long is an Online Agent Authorisation code valid?

HMRC’s Self Assessment Manual says the code expires if it is not entered within 28 days.

Can a client authorise me through their business tax account?

Yes, for some taxes such as PAYE and CIS. The client needs your Agent Government Gateway Identifier, found under “Authorise client” in your HMRC online services for agents account.

Can a supporting agent submit an MTD client’s tax return?

No. Only the main agent can finalise the tax position and submit the return.

Sources

Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance on 6 November 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.