Mandatory HMRC registration for tax advisers begins as roll-out opens

A new requirement for paid tax advisers to register with HMRC began rolling out on 18 May 2026, with unregistered agents eventually unable to act for clients.

What is changing

HMRC has opened registration for a new mandatory regime covering tax advisers, the formal name for which is Modernising and Mandating Tax Adviser Registration, or MMTAR. From 18 May 2026, anyone who is paid to interact with HMRC on behalf of clients is required to register, through a single digital system intended to replace a patchwork of earlier processes. A new interactive checker tool on GOV.UK lets advisers work out whether the rules apply to them and what they need to do next.

The policy was first announced at Budget 2025, following a public consultation in 2024, and the government says respondents were strongly in favour. HMRC frames it as a way to make advisers easier to identify, raise standards across the tax advice market, and support agents who already play by the rules. The department is investing £36 million to modernise the registration system as part of its Plan for Growth, and underlying guidance was published on GOV.UK on 17 February 2026.

Who must register, and when

Registration is being introduced gradually, with different groups invited to register in separate windows running from 18 May 2026 through to 31 March 2027:

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Advisers will have three months from the start of their window to apply for an agent services account, and may continue acting for clients during that period and while HMRC considers their application. Those who already hold an agent services account do not need to register again; HMRC says it will contact them through that account if it needs further information.

What advisers need to do

HMRC defines a tax adviser broadly: if a person interacts with the department about someone else's tax affairs and is paid for it, they are in scope, including advisers based overseas who act for UK taxpayers. To register, an adviser will typically need their Government Gateway credentials, their firm's Unique Taxpayer Reference and associated postcode, any company and VAT registration numbers, identity details for sole traders or partnerships, and details of their anti-money laundering supervisory body. Organisations that do not provide tax advice as a business, such as voluntary sector bodies, are not required to register, and other carve-outs, such as customs intermediation, are set out in the guidance.

Sanctions for non-compliance

The regime has teeth. Eligible advisers who miss their deadline, or who fail to meet HMRC's required standards, will not be permitted to interact with the department on behalf of clients. Where an adviser continues to act without registering, HMRC says it may apply sanctions, including financial penalties if they keep interacting after being told to stop. Beyond the formal penalties, the department points out that failing to register could disrupt services for clients and damage trust with the individuals and businesses relying on that adviser.

Implications

For the UK's tax advice market this is a structural change rather than an administrative tweak. Registration becomes a gateway to acting for clients at all, and the phased timetable means firms need to identify their own window now rather than treat 18 May as a single hard deadline. The practical risk is timing: leaving an application to the end of a window invites processing delays that could interrupt client work.

It also signals where HMRC is heading. Tying the ability to represent clients to registration and minimum standards gives the department a lever it has not previously had over the adviser population, and links directly to its broader agenda of raising standards and closing the tax gap. Advisers and the firms that employ them would be wise to check the GOV.UK checker tool early and confirm both their registration window and that they meet the conditions.