What is MMTAR?
From 18 May 2026, the UK introduced a new statutory requirement for tax advisers (those paid to interact with HM Revenue & Customs on behalf of clients) to register under the Modernising and Mandating Tax Adviser Registration (MMTAR) scheme. (gov.uk)
The system requires registration via an Agent Services Account (ASA) and sets out minimum standards advisers must meet. Exemptions apply (e.g. if not acting as a business or in certain roles). (gov.uk)
When and Who Must Register (Phases)
Registration is phased across different groups, each with its own window:
| Phase | Adviser Type | Registration Window |
|---|---|---|
| Phase 1 | New tax advisers or those interacting with HMRC without ASA, Self Assessment or Corporation Tax account | 18 May – 18 August 2026 (gov.uk) |
| Phase 2 | Advisers with Self Assessment or Corporation Tax account, but no ASA | 18 August – 18 November 2026 (gov.uk) |
| Phase 3 | Advisers offering only payroll services | 18 November 2026 – 18 February 2027 (gov.uk) |
| Phase 4 | Those with ASA already, financial services firms, others defined in secondary legislation | 31 December 2026 – 31 March 2027 (gov.uk) |
What You Need to Do
- Check if you meet HMRC’s definition of a tax adviser (paid interaction with HMRC about others’ tax affairs). If yes, you’ll need to register. (gov.uk)
- Set up an Agent Services Account (ASA) if you don’t already have one.
- Prepare required information: Government Gateway ID, Unique Taxpayer Reference (UTR), VAT or company registration numbers if applicable, National Insurance/Date of Birth for identity verification. (gov.uk)
- Use the on-GOV.UK interactive checker to confirm whether you need to register and in which window.
Risks of Not Registering
- From the end of your registration window, unregistered advisers interacting with HMRC on behalf of clients are not permitted to act.
- Potential sanctions including financial penalties and suspension.
- Disruption for clients due to inability to deal with HMRC, loss of trust.
Practical Example
John, a sole trader providing tax advice, not yet registered. He contacted HMRC on behalf of clients in April 2026 (before registration opened). His registration window opens 18 May – 18 August 2026 (Phase 1). He has three months from 18 May to register and can continue acting during that period. If he fails to register by 18 August, he must stop acting as an adviser to clients (unless exempt).
Tips to Prepare
- Gather all required personal and business documentation in advance.
- Use HMRC’s guidance and stakeholder packs.
- Review whether your role or services are exempt to reduce unnecessary effort.
- Inform clients of changes so expectations are managed.
Conclusion
MMTAR is a major compliance shift for tax advisers in the UK. Those affected must ensure they register in the correct window, meet the required standards, and stay informed about updates in secondary legislation. Act early to avoid sanctions and maintain continuity.