What is the MMTAR rule?
The UK government has introduced MMTAR (Modernising and Mandating Tax Adviser Registration). As of 18 May 2026, any tax adviser who receives payment to interact with HMRC on behalf of clients must register online under this new scheme. This replaces previous agent registration processes, aiming to simplify the system and raise professional standards. (gov.uk)
Who needs to register—and when?
MMTAR rolls out in phases. You will need to register if you fall into one of these categories, during the specified window:
| Time Period | Who Must Register |
|---|---|
| 18 May-18 August 2026 | New tax advisers and those interacting with HMRC but without any Agent Services Account (ASA), Self Assessment, or Corporation Tax account. (gov.uk) |
| 18 August-18 November 2026 | Advisers who have a Self Assessment or Corporation Tax account but no ASA. (gov.uk) |
| 18 November 2026-18 February 2027 | Advisers whose only business is payroll services. (gov.uk) |
| 31 December 2026-31 March 2027 | Advisors already holding an ASA and financial services organisations. Exact definitions to be provided. (gov.uk) |
How to register: Key Requirements
You'll need to prepare the following to complete registration:
- Government Gateway user credentials (ID and password)
- Your firm’s UTR and associated postcode
- Company registration number, if applicable
- VAT registration number, if applicable
- National Insurance number and date of birth (for sole traders, partnerships, or LLPs)
- Details of your Anti-Money Laundering supervisory body (name, membership, renewal date)
Note: Advisers acting informally (e.g., unpaid community advice) may be exempt. Custom and excise intermediation activities may also have exemptions. (gov.uk)
Risks of non-registration & action steps
Failure to register by your deadline means you cannot legally interact with HMRC in your adviser role—and you may face sanctions, including loss of ability to act and financial penalties. (gov.uk)
What you can do now:
- Determine which phase applies to you based on your current registration status.
- Visit GOV.UK to use their interactive checker tool to confirm whether registration applies to you. (gov.uk)
- Gather required documents (UTR, NI, etc.) ahead of your registration window.
- If you use agents or have clients who rely on you, inform them about changes that may affect the continuity of your representation of them with HMRC.
Practical example
Sarah is a self-employed tax adviser who helps small business clients with Corporation Tax filings and has a self assessment account but does not yet have an ASA. She must register between 18 August and 18 November 2026. If she had only payroll service clients, her window would be 18 November 2026 to 18 February 2027. If she fails to register within her window, she may no longer represent clients.
Making sure you comply with MMTAR isn’t just about following rules—it protects clients, boosts credibility, and maintains trust with HMRC. Get on top of it early to avoid last-minute scramble or service disruption.