There is a quiet change happening to the UK tax profession this year. From 2026, if someone is paid to deal with HMRC on your behalf, they have to be registered with HMRC to do it. It is a new legal requirement, it is being switched on in stages, and the first stage has already closed its doors.
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.
What the rule actually says
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".
everyone has a different deadline
The 18 August date has been reported as though it applies to the whole profession. It doesn't. 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, not 18 August 2026. 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
Two things.
First, financial penalties and sanctions for the adviser. 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, not a case study of a real client.
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 — not out of defiance, but because the letter looked like something 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.
A registration failure by your adviser does not become HMRC's problem, and it does not become a reasonable excuse. It becomes a week of your life and, potentially, the real 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. A vague answer — or a first-time-hearing-about-it answer — tells you something useful. 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. Write the list down.
What this doesn't change
Be clear-eyed about what registration is. It is a register, not a qualification. It does not mean HMRC has assessed whether your adviser is any good, and it should not replace the checks that actually tell you that: professional body membership, professional indemnity insurance, and whether they will explain a number to you until you understand it.
It also doesn't change a single one of your own deadlines. Your VAT dates, your corporation tax payment date, your Self Assessment, your Making Tax Digital obligations — all unmoved.
What it does do is make one thing visible that used to be invisible. Until now, there was no straightforward way to establish whether the person filing your returns was recognised by HMRC as anything at all. From 2027, when the last window closes, they can't.
Where we sit
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 asked question one above and didn't get a straight answer, that is usually a symptom of something broader than one registration deadline. Moving accountant is less painful than most owners assume — the handover is largely paperwork between the two firms. And if you would rather have your tax work, your payroll and your filings sitting with one firm, that is what our packages are built around.
Either way, ask the three questions.