What is MMTAR?
“Modernising and Mandating Tax Adviser Registration” (MMTAR) is a Government policy under which tax advisers paid to interact with HMRC on behalf of clients must register with HMRC and meet minimum standards. (gov.uk)
What’s Changing & When?
The rollout is phased, each group having its own registration window:
- 18 May–18 August 2026: New advisers or those without Previous tax/account registrations with HMRC. (gov.uk)
- 18 August–18 November 2026: Advisers with Self Assessment or Corporation Tax accounts but no Agent Services Account (ASA). (gov.uk)
- 18 November 2026–18 February 2027: Advisers who provide only payroll services. (gov.uk)
- 31 December 2026–31 March 2027: Those who already have an ASA and financial services organizations. Secondary legislation will define who qualifies. (gov.uk)
Why This Matters
Only registered tax advisers will be permitted to interact with HMRC on behalf of taxpayers. Failure to register by your group’s deadline could mean:
- being blocked from filing returns, appeals, error corrections.
- clients being unable to claim through you.
This also includes restrictions: advisers with promoter penalties or cases of non-compliance may be barred from registering. (assets.publishing.service.gov.uk)
What Advisers Need to Do Now
- Check whether registration applies to you: see HMRC’s interactive checker.
- Identify your registration window (2026–2027)
- Prepare supporting documents: For individuals, ASA registration will require your UTR number, National Insurance number, and identity proof. Firms may need company or VAT numbers. (gov.uk)
- Apply for an ASA online when your window opens. There’s a 3-month grace period where you can still act while HMRC processes your application. (gov.uk)
Implications & Advice
If you're a tax adviser:
- Late registration could disrupt your ability to serve clients.
- Clients should verify you are registered to avoid issues with late filings or penalties.
If you're a taxpayer who uses advisers:
- Ask whether your adviser is registered under MMTAR.
- Be cautious — unregistered advisers could face restrictions or penalties.
Case Study Example
Jane is a specialist adviser handling only payroll services and has no other HMRC agent accounts. Her registration window is 18 November 2026–18 February 2027. If she doesn’t register by early February 2027, she may no longer be able to interact with HMRC to file payroll-related submissions for clients.
Key takeaway: tax professionals must map their own timeline and clients must confirm adviser registration. MMTAR marks a major change in standards and accountability in UK tax advice.