If you're working with an accountant or R&D tax specialist to file your claim, you now have a fair reason to ask whether that adviser is actually allowed to represent you.
From May 2026, HMRC introduced Mandatory Tax Adviser Registration (MMTAR), a new requirement for anyone who deals with HMRC on a client's behalf for payment. It isn't just an adviser's problem to sort out quietly in the background. If the person filing your R&D tax credit claim loses the right to interact with HMRC, your claim is affected too.
Does your adviser need to be registered with HMRC?
From May 2026, anyone who deals with HMRC on a client's behalf for payment must register for an Agent Services Account (ASA), unless a specific exemption applies. That covers the ways an adviser typically interacts with HMRC on your behalf, including phone calls, post, email, and filing returns or claims such as your Additional Information Form required for the R&D tax credit.
An adviser who doesn't register by their deadline, or who doesn't meet HMRC's registration conditions, won't be permitted to interact with HMRC on a client's behalf at all. If your R&D tax adviser is filing your claim and loses the right to act, that claim can stall, often at a point where you have little visibility into what's happening or why.
MMTAR replaces a patchwork of older registration processes with a single, streamlined digital system. HMRC's reasoning is that a more consistent register makes advisers easier to identify, raises the baseline standard across the market, and gives taxpayers a clearer picture of who's actually authorised to represent them.
With the R&D tax credit scheme previously having a reputation for attracting underqualified advisers looking to profit off the scheme’s minimal regulation, MMTAR is a welcome change for many claimants looking for more security.
Who needs to register, and by when
Any tax adviser who interacts with HMRC on behalf of a taxpayer must be registered. Your adviser must be registered if they:
- Advise you in relation to tax
- Act as agent on your behalf in relation to tax
- Prepare any document that HMRC is likely to rely on regarding your tax position
There are some exceptions, like those providing free or voluntary tax advice (i.e., not in the course of business), in-house tax advisers, or insolvency practitioners.
This includes:
- Accountants acting as sole traders
- Accountancy firms
- An agent who submits simple repayment claims
- Financial advisers or wealth managers
- Multi-disciplinary professional services firm where tax advice services are provided to clients
- Boutique firms advising on specific tax measures, like R&D tax credit advisers
Registration is rolling out in four phased windows. Which one applies to your adviser depends on their current setup with HMRC.
|
Registration window |
Who it applies to |
|
18 May – 18 August 2026 |
New advisers, or those without an ASA, Self Assessment or Corporation Tax account |
|
18 August – 18 November 2026 |
Advisers with a Self Assessment or Corporation Tax account, but no ASA |
|
18 November 2026 – 18 February 2027 |
Advisers who solely provide payroll services |
|
31 December 2026 – 31 March 2027 |
Advisers who already hold an ASA, and financial services organisations |
If your adviser already holds an Agent Services Account, they don't need to register again. HMRC contacts them directly through that existing account if it needs more information to move them onto the new system.
Registration itself is free, and advisers can continue working for clients throughout the registration period, and while HMRC considers their application, so a registration in progress isn't, on its own, a reason for your claim to stop moving.
Is there a public register you can check?
Not really. HMRC has published an interactive checker tool for advisers, but it's built for advisers to work out their own obligations, not for clients to look up a specific firm.
There's currently no public, client-facing register where you can search for your adviser's name and see their registration status. That means asking directly the only route to find out if your adviser is registered.
Three questions worth asking your R&D tax adviser
If you want a straight answer rather than an assumption, these three questions cover what matters:
- Do you already hold an Agent Services Account?
- If not, which registration window applies to you, and when will you register?
- Can you confirm once you've registered, and what happens to my claim in the meantime?
What happens if an adviser doesn't register in time
If an adviser continues to interact with HMRC after being told to stop, HMRC can apply sanctions, including financial penalties for the adviser.
For you as the client, the more immediate consequence is practical: a claim that's paused, delayed, or handed to someone else while your adviser sorts out their registration.
This is a compliance change aimed at raising standards across the market, not a sign that most advisers are at risk. HMRC introduced MMTAR to make advisers easier to identify and hold to consistent standards, following a public consultation in which respondents were strongly in favour of the change. Most established R&D tax specialists will register within their window without any disruption to the clients they work with.
Whether your claim currently sits with an adviser, or you're still choosing one, you should know if they’re registered before you hit the submit button. Get in touch and the Tax Cloud team can talk you through where things stand.
