The second registration window under HMRC's Modernising and Mandating Tax Adviser Registration programme opened this week. Advisers who already hold a Self Assessment or Corporation Tax agent account but do not yet have an agent services account have until Wednesday 18 November 2026 to register. Registration is free, it is online, and it takes minutes.
This is being written up everywhere as an adviser story, which is why most business owners will scroll past it. It is not an adviser story. HMRC's own guidance puts the consequence plainly: if a firm needs to register and does not, it will not be able to interact with HMRC on behalf of its clients. The registration is your accountant's job. The returns that stop going in are yours, and so are the penalties. That is the whole reason this belongs in front of owners rather than in a practitioner newsletter.
What is actually changing
MMTAR replaces a scatter of older agent registration processes with a single digital route built on the agent services account. The definition of who is caught is deliberately wide. HMRC's test is that if you interact with HMRC about someone else's tax affairs and get paid for it, you are a tax adviser — and interaction includes phone, post, email, messages through GOV.UK or the HMRC app, making payments, and sending returns, claims or other documents.
You are caught even if you do not think of yourself as a tax adviser, even if tax is not your main business function, and even if you act for only one client. The bookkeeper who files your VAT return is in scope. So is the payroll bureau, on a later date. It is the legal entity that registers, not individual staff, although HMRC will run checks on named people inside the business.
The rollout runs in stages. Firms with no prior agent code registered from 18 May 2026. Firms with an existing Self Assessment or Corporation Tax account are in the window that opened on 18 August 2026 and closes 18 November 2026. Firms that only provide third-party payroll services register from 18 November 2026. Financial services organisations register from 31 December 2026. Each window runs three months, and a firm can keep acting for clients during those three months and while HMRC considers the application.
The Northern Ireland part nobody is covering
A UK-wide HMRC deadline usually reads the same in Newry as it does in Nottingham. This one does not, for two reasons that sit in the guidance and have gone unremarked.
First, your adviser may be an overseas tax adviser. HMRC states that you must register if your business is based outside the UK, and that overseas firms must use the existing registration route rather than the new online service. On top of that, HMRC will require authenticated evidence that the firm meets the registration conditions — documents notarised by a qualified independent notary or equivalent, and translated into English where needed. HMRC has said not to send it yet and that it will ask when it is needed.
Northern Ireland is the only part of the United Kingdom with a land border, and using an accountant in Dundalk, Monaghan or Letterkenny is ordinary practice here, not an exotic arrangement. Every one of those firms is an overseas tax adviser in HMRC's language. They face a slower route in, a notarisation step no Belfast firm has to think about, and a document requirement whose timing HMRC has not yet fixed. If your accountant is south of the border, the 18 November date is the start of their problem rather than the end of it.
Second, there is a Northern Ireland carve-out sitting in the exemptions, and it is easy to misread. HMRC's list of who does not need to register includes anyone who only interacts with HMRC on customs or import VAT, as a VAT representative, as an intermediary for the Import One Stop Shop scheme, and — named explicitly — as a Northern Ireland tax representative, appointed by a business outside the UK to manage certain Northern Ireland-specific tax obligations.
Four exemptions, and four things NI traders use routinely because of the Windsor Framework. The trap is that the exemption follows the work, not the firm. If one practice handles both your customs declarations and your Company Tax Return, the customs half is out of scope and the tax half is in scope, and it still has to register. A firm that reads the exemption list, sees itself in it and concludes it has nothing to do is exactly the firm that gets locked out in November.
Worked example: an 18-person engineering company in Newry
Take an illustrative Co Down precision engineering company — not a client, with round figures so you can follow the arithmetic and swap in your own. Eighteen employees, VAT-registered filing quarterly, 31 December year end, two director-shareholders who each file a Self Assessment return. Its accountant is twenty minutes down the road in Dundalk, and does not complete registration in time.
HMRC stops accepting agent interaction. The company spends four months finding a registered firm and getting authorisations back in place. Nothing dramatic happens. Filings simply slip.
- Payroll. With 10 to 49 employees, the monthly late filing penalty is £200. The first failure in a tax year is not charged, so four late months costs three penalties — £600.
- Company Tax Return. £200 the day after the deadline and another £200 at three months — £400.
- Self Assessment, two directors. £100 each straight away, then £10 a day once the return is three months late. Four months late is roughly 30 days of daily charges, so £400 each — £800.
- VAT. Two late quarterly returns bank two penalty points against a threshold of four. No cash penalty yet — but points persist, and once at the threshold every subsequent late return is £200.
That is £1,800 of hard penalties, on a registration that costs nothing and takes minutes. And it is the mild version. Let the Company Tax Return reach six months late and HMRC raises a tax determination you cannot appeal, plus a penalty of 10% of the unpaid tax. On a £46,000 corporation tax liability that is £4,600, taking the total to £6,400 — and another 10%, a further £4,600, at twelve months. Three consecutive late Company Tax Returns turn the £200 penalties into £1,000 each.
None of that is a penalty for your adviser's paperwork. It is the ordinary cost of returns going in late, and the filing obligation was always yours.
What this means for a Northern Ireland owner
If your accountant is in Northern Ireland or Great Britain, this is a one-question conversation. Ask whether they hold an agent services account and when their window closes. A registered firm answers in a sentence.
If your accountant is in the Republic, ask the same question and then ask a second one: whether they have started the overseas route and what they have been told about notarised documents. Their registration involves a step yours does not, on a timetable HMRC has not published. This is the same cross-border asymmetry that runs through VAT on cross-border trade — the rule is UK-wide, the friction is ours.
If you handle your own returns, none of this touches you. If you pay anyone at all to speak to HMRC for you — including a bookkeeper who only files VAT — it does.
Two things worth doing this week
- Ask your adviser the direct question, in writing. Not "are you aware of MMTAR" but "do you hold an agent services account, and when does your registration window close?" There is no public register to check this against, so asking is the only route. If the answer is anything other than a clear yes, diarise a check for the first week of November. HMRC's guidance on if and when you need to register as a tax adviser includes an interactive checker your adviser can run in a couple of minutes, and the answer to your question should take them about as long.
- Check who else speaks to HMRC for you. Most owners have more than one agent and have never counted them — an accountant for the accounts, a bookkeeper for VAT, a payroll bureau, sometimes a separate customs agent. Each is a separate legal entity with its own registration window and its own deadline. Payroll-only providers are not due until 18 November, and financial services organisations not until 31 December, so the answers will legitimately differ. The conditions each has to meet, including anti-money laundering supervision, are set out on HMRC's registration conditions page.
What is still uncertain, and when you will know
The overseas evidence process is not built yet. HMRC has confirmed that notarised and translated documents will be required from overseas advisers and relevant individuals, but has explicitly told firms not to send anything until asked. For a Dundalk practice with Newry clients, that is a known future requirement on an unknown date. Until HMRC publishes it, no cross-border firm can tell you it is fully done — only that it has applied.
What enforcement looks like in practice. HMRC says it may limit an adviser's ability to act, and that firms operating without registering could face enforcement action including financial penalties. It has not said how quickly access is withdrawn after 18 November, and it has confirmed that advisers who apply and receive a registration number can continue engaging with HMRC while the application is processed, with online services not affected in the short term. So a late application is not the same as a missed one. A firm that has not applied at all by 18 November is the exposure.
The payroll and financial services windows. These open on 18 November 2026 and 31 December 2026 and run three months each, so the same question needs asking again in the new year of any provider that only does your payroll. Diarise it now rather than reading about it in February.
Where we come into this
There is no drama in this story and nothing to panic about. There is a free, ten-minute administrative step that a firm either takes or does not, and a set of consequences that fall entirely on the client if it does not. The businesses that get hurt will not be the ones with a bad accountant. They will be the ones who assumed somebody had it in hand and found out in December.
For what it is worth, you are entitled to ask us the same question, and the answer to who licenses us, what we are insured for and what we are not permitted to do is set out in full on our regulatory information page rather than on request. Our payroll and pensions and Making Tax Digital work is where filing deadlines get met rather than explained afterwards, and our Ballymena office covers the whole of Northern Ireland — including the cross-border detail a firm in Great Britain has no particular reason to have read.