From 2026, anyone paid to deal with HMRC on your behalf has to be registered with HMRC. The requirement is being introduced in stages, and the first registration window has already closed.
HMRC calls it Modernising and Mandating Tax Adviser Registration, or MMTAR. On 20 July it published a reminder that tax advisers had one month left to register under the new rules before the first deadline on 18 August 2026.
If you are a business owner, there are three questions worth asking whoever files your returns — ideally before the autumn.
Who has to register
The scope is broad on purpose. HMRC's position is that anyone paid to interact with HMRC on behalf of someone else about their tax affairs is generally a tax adviser unless an exemption applies. That is not limited to chartered accountants and tax lawyers. It catches bookkeepers who file VAT returns, payroll bureaux submitting RTI, the local one-man band doing forty Self Assessments each January, and advisers based overseas who act for UK taxpayers.
A handful of things sit outside it — voluntary sector organisations that aren't providing tax advice as a business, and some specific activities such as customs intermediation.
Registration itself is free and done online. The government has put £36 million behind the programme. Robert Jones, HMRC's Director of Intermediaries, framed it this way: the new requirements "will help create a fairer, more transparent tax advice market".
The four registration windows
The 18 August date applies only to the first group of advisers. Registration opened on 18 May 2026 and rolls out in windows through to 31 March 2027, and which window your adviser sits in depends on what HMRC accounts they already hold.
- 18 May – 18 August 2026. New advisers, and anyone dealing with HMRC without an agent services account (ASA), Self Assessment account or Corporation Tax account. This is the window that has just closed.
- 18 August – 18 November 2026. Advisers with a Self Assessment or Corporation Tax account but no ASA.
- 18 November 2026 – 18 February 2027. Payroll-only service providers.
- 31 December 2026 – 31 March 2027. Advisers who already hold an agent services account, plus financial services organisations.
An adviser gets three months from the point their window opens to get registered, and can carry on dealing with HMRC during that window and while their application is being looked at.
If your accountant is an established firm filing your VAT and corporation tax through an agent services account, their deadline is 31 March 2027. Nothing is wrong if they haven't registered yet. Their window hasn't opened.
The people affected by the first two deadlines are, broadly, the smaller and newer end of the market: the sole practitioner, the bookkeeper who took on tax work, the adviser who has always filed through their client's own HMRC login rather than as a proper agent.
What happens to an adviser who doesn't register
First, the adviser faces financial penalties and sanctions. Second — and more disruptive — HMRC can stop dealing with them. An adviser who keeps acting after being told to stop can be prohibited from interacting with HMRC at all. HMRC's own warning in the 20 July notice is aimed squarely at clients: failing to register "could delay or disrupt services for clients and damage trust with individuals and businesses relying on professional tax support".
Here is how that plays out in practice. This is an illustration of the sequence, put together for this article.
Imagine a small limited company with a 31 March year end, VAT registered, quarterly returns filed by a self-employed bookkeeper who has looked after the books for six years and files through the company's own HMRC credentials. The bookkeeper never had an agent services account, so they fell in the first window and needed to be registered by 18 August. They didn't register, because they assumed the letter was aimed at big firms.
- The company's VAT quarter ends 30 September. The return is due 7 November.
- By the time the return is being prepared, the bookkeeper is unregistered and outside their window.
- The disruption is a filing that has to be done another way, at short notice, by someone else, in the week it is due.
- The company is still liable for the return and any late-filing consequences. HMRC's rules bind the adviser; the obligation to file on time never stopped being the company's.
Your adviser failing to register is not a reasonable excuse, so the company can face the cost of a late VAT return.
Three questions to ask this week
None of these require you to understand the regime. Send them in an email and read the answers.
1. "Which registration window are you in, and are you registered or waiting?" A good answer is specific: we hold an agent services account so our window is 31 December to 31 March, or we registered in June. Anyone paid to touch your tax affairs should know their own date by now; HMRC has been publishing on this since May.
2. "Are you filing as an agent, or using our HMRC login?" Filing through a proper agent services account is the normal professional set-up and it is also the marker for the last registration window. If someone is filing your returns using your own Government Gateway credentials, that is worth understanding for its own sake — separately from any of this — because it means HMRC has no record of an agent acting for you at all.
3. "Who else touches our HMRC filings?" Most owners under-count. Payroll is often a different provider from the accounts. VAT is sometimes a bookkeeper. A one-off R&D or CIS specialist may have been involved. Every one of them is in scope of these rules, and payroll-only providers have their own window from 18 November.
What registration tells you about an adviser
Registration records that an adviser is known to HMRC. It is a register, and HMRC does not assess the adviser's quality. To judge that, check professional body membership, professional indemnity insurance, and whether they will explain a number to you until you understand it.
Your own deadlines stay the same: your VAT dates, your corporation tax payment date, your Self Assessment and your Making Tax Digital obligations.
From 2027, when the last window closes, anyone filing your returns for a fee will have to be registered with HMRC.
Buzz's registration window
We hold an agent services account and file as an agent for our clients, which puts Buzz in the final window closing 31 March 2027. We are tracking it as a dated item on our own compliance calendar, and our clients don't need to do anything.
If you are thinking of moving, switching accountant is largely paperwork between the two firms. Our accountancy packages put your tax work, your payroll and your filings with one firm.