HMRC put out a reminder on 20 July that the first registration window under Modernising and Mandating Tax Adviser Registration closes on 18 August 2026. If you run an established practice with an agent services account, the most useful thing anyone can tell you this week is that this is almost certainly not your deadline. Yours opens on 31 December. Reading the timetable properly will save you an afternoon, and — more to the point — stop you filing the whole thing under "done" when the part that actually affects your firm has not started yet.
The deadline is the small story. The larger one is what the four-phase timetable says about the direction of travel: government has decided that anyone paid to deal with HMRC on somebody else's behalf should be identifiable, condition-checked and, if they fall short, removable. Tax advisers go first. Financial services organisations are in the final phase, from 31 December 2026. By 31 March 2027, both professions that sit around a business owner's affairs will have been through the same registration gate. That is a quiet but real change in the plumbing underneath how accountants and financial planners work alongside each other, and it is worth understanding before it arrives rather than after.
What MMTAR is, and what closes on 18 August
Modernising and Mandating Tax Adviser Registration was announced at Budget 2025, following public consultation in 2024. HMRC's guidance was first published on 17 February 2026 and the online service opened on 18 May 2026. Registration is free, done online, and replaces a range of previous processes with a single route into an agent services account. The government is putting £36 million into the modernisation as part of its Plan for Growth.
The rollout runs in four phases, and HMRC has published them as follows:
18 May to 18 August 2026 — new tax advisers, or advisers interacting with HMRC without an agent services account, Self Assessment or Corporation Tax account.
18 August to 18 November 2026 — advisers with a Self Assessment or Corporation Tax account, but without an agent services account.
18 November 2026 to 18 February 2027 — advisers who solely provide payroll services.
31 December 2026 to 31 March 2027 — those who already have an agent services account, and financial services organisations.
Each group gets three months from the opening of its window to apply, and HMRC's guidance confirms that a firm can carry on interacting with HMRC for clients during those three months and while its registration is being considered. Robert Jones, HMRC's Director of Intermediaries, framed the purpose as creating a fairer, more transparent tax advice market and giving taxpayers greater confidence in the advice they receive.
So the 18 August date belongs to a narrow group. If your practice has held an agent services account for years, you are not in it, and HMRC's guidance says plainly that you do not need to register again at this stage — HMRC will contact you through the account itself if it wants more information.
The firms that are in phase one, and usually do not realise it
Here is where a generic reading of the headline goes wrong. Phase one is not only about brand-new practices set up by someone leaving industry. It is about any legal entity that interacts with HMRC for paying clients without already holding an agent services account, Self Assessment or Corporation Tax agent code. HMRC's guidance is explicit that it is the legal entity which registers, not the individual employees.
Take a practice group that looks entirely ordinary. The main trading company has held an agent services account since 2019. Eighteen months ago the partners incorporated a second company to house a specialist R&D and capital allowances team, and this spring they spun the bookkeeping and payroll work into a third. Three legal entities, one letterhead, one client base. Under the published timetable those three entities can sit in three different windows: the original company in the phase running from 31 December 2026, the specialist company in phase one if it has been dealing with HMRC in its own name without its own account — the window closing on 18 August — and the payroll company in the window from 18 November 2026 if payroll on behalf of clients is genuinely all it does.
Nobody sets a group up to be awkward. But the registration duty follows the entity, and firms that have restructured, incorporated an LLP, or created a subsidiary for a service line in the past two years are exactly the ones most likely to have an entity in phase one while assuming the whole group is covered by the parent's long-standing account. That is the specific thing worth checking before 18 August, and HMRC publishes an interactive checker tool for precisely this question.
The definition of who is caught is also broader than the instinct. HMRC treats you as a tax adviser if you interact with HMRC about someone else's tax affairs and are paid for it — where interaction includes phone, post, email, messages through the GOV.UK website or HMRC app, making payments, and sending returns, claims or other documents. The guidance states that registration is required even if you do not view yourself as a tax adviser, even if you are a registered sole trader, even if tax is not your main business function, and even if you act for a single client.
The conditions are about your firm, not your clients
The part of this regime that deserves more attention than the deadline is what a firm has to satisfy to get through it. HMRC's registration conditions require evidence that the business is supervised for anti-money laundering — a digital copy of the supervision certificate or a confirmation email will do. Beyond that, the business must not have relevant outstanding tax returns or unpaid tax unless covered by a payment plan; must not be subject to an HMRC decision refusing to interact with it, an anti-avoidance sanction or a stop notice; must not have relevant unspent convictions for fraud or tax offences; must not be formally insolvent; and must not be suspended or permanently banned from registering.
HMRC also names people. Firms with five officers or fewer must declare all of them as "relevant individuals" whether or not they touch tax work. Firms with six or more identify those who make strategic or management decisions about the tax advice work, and if that produces fewer than five, must nominate additional officers to reach five. Relevant individuals do not provide anti-money laundering evidence themselves, but they must not be disqualified from acting as a director, in the UK or overseas.
A practice can spend a decade being meticulous about client compliance and still be tripped up by its own. The registration conditions ask about your firm's returns, your firm's supervision and your firm's officers.
That inversion is the genuinely new thing. Continued access to HMRC on behalf of clients now depends on the firm's own house being in order. Where an adviser misses their window, HMRC's position is that they will not be permitted to interact with HMRC for clients, and that continuing to act after being told to stop can attract sanctions including financial penalties.
Why the fourth phase matters to joined-up client work
The final window, from 31 December 2026 to 31 March 2027, covers two groups at once: firms that already hold an agent services account, and financial services organisations. HMRC's May announcement said the full definition of that second group would follow via secondary legislation; by the July release, HMRC stated that the definition for the group had already been published.
Set alongside that, the guidance carries an exemption that anyone working across the two professions should read carefully. A business does not need to register where it interacts with HMRC for someone else because the law requires it to, even if it is paid — HMRC gives insolvency practitioners and some pension or investment firms as examples. So the position for a financial services firm is not a single blanket answer, and an accountant should not assume one way or the other about a planner they work with.
This is precisely the sort of detail that gets guessed at rather than established. Two professionals sit around the same client. Each assumes the other's regulatory position is roughly like their own, or roughly like it was five years ago. Neither asks, because asking feels like a challenge rather than housekeeping. It is the same failure mode we described in who does what: the boundary is usually clear in principle and vague in the specific instance, and the vagueness is what stops good work happening.
What a joined-up conversation looks like off the back of this
Nothing here changes what an accountant can and cannot say. Noticing that a client has idle cash on the balance sheet, no protection behind a personal guarantee, or a sale in motion is observation, and it is the accountant's to make. Recommending a particular pension, product or investment is a regulated activity and belongs with an authorised firm. Those are the six signals set out in the signals already in your client file, and MMTAR does not move that line by an inch.
What it does change is the surrounding context, in a useful direction. When both professions have been through the same registration gate, "who is registered to do what for this client" becomes a question with a short, factual answer rather than an awkward one. A handover between an accountant and a planner works best when each side can state their own position plainly — what they are supervised for, what they are registered to do, where their responsibility stops — and the client can see the join. The mechanics of that handover are set out in the anatomy of an introduction.
Three things to do this week
First, list every legal entity in your group and run HMRC's checker against each one separately. Not the practice, not the brand — the companies, the LLPs and any sole trader registration. If a newer entity has been contacting HMRC in its own name without its own agent services account, its window closes on 18 August 2026.
Second, find your anti-money laundering supervision certificate and confirm it is current and available digitally. It is a registration condition, HMRC wants a digital copy or a confirmation email, and lapsed or unlocatable supervision is a slow problem to fix under time pressure.
Third, do the relevant individuals exercise now rather than in the registration form. Count your officers, apply the five-or-fewer and six-or-more rules, and check that everyone on the resulting list would satisfy the conditions — including the requirement not to be disqualified as a director anywhere.
What is not yet settled
Three things honestly remain open. The overseas evidence process is not live: HMRC's guidance tells overseas advisers their documents must be notarised and translated, but also says not to send evidence yet and that HMRC will tell them when. The sanctions regime has published guidance but no track record, so how HMRC exercises it in practice will only become clear once the later windows close. And the interaction between the fourth phase's two groups — existing account holders and financial services organisations — is the part firms operating across both worlds will want to watch through the autumn, ahead of that window opening on 31 December 2026.
The wider point about who gets a conversation at all
The lang cat's State of Advice Report 2025 found that just 9% of UK adults had paid for financial advice in the preceding two years, while 91% of those who did take advice found it valuable. It also put the average age of an IFA client at 59, and the average new client portfolio at £411,000. Those figures describe a profession serving a narrow group extremely well and reaching very few people outside it. We set that argument out in full on the advice gap.
Registration regimes do not close that gap. They make it easier to tell who is standing where, which is a precondition for something better rather than the thing itself. The gap closes when the professional a business owner already trusts notices something in the numbers and says so out loud. Everything else — registers, accounts, conditions, windows — is scaffolding around that conversation.
Common questions
Our practice already has an agent services account — is 18 August our deadline?
No. HMRC's guidance is explicit that if you already have an agent services account you do not need to register again at this stage. The window closing on 18 August 2026 covers new tax advisers, and advisers interacting with HMRC without an agent services account, Self Assessment or Corporation Tax account. Firms that already hold an agent services account fall into the final phase, which runs from 31 December 2026 to 31 March 2027. HMRC has said it will make contact through the agent services account itself if it needs more information to move a firm across to the new digital system, so the practical action for an established practice is to make sure somebody actually reads messages arriving there.
What counts as interacting with HMRC on a client's behalf?
Wider than most firms assume. HMRC's guidance says that if you interact with HMRC about someone else's tax affairs and are paid for it, you are treated as a tax adviser. Interaction expressly includes phone, post or email, messages sent through the GOV.UK website or the HMRC app, making payments, and sending returns, claims or other documents. The guidance goes further and states you need to register even if you do not view yourself as a tax adviser or describe your work as tax advice, even if you work as a registered sole trader, even if helping people with tax is not your main business function, and even if you act for only one client.
Which window applies to a firm that also runs payroll for clients?
It depends on whether payroll is all you do for them. HMRC's phased timetable gives a separate window of 18 November 2026 to 18 February 2027 to advisers who solely provide third-party payroll services and do not interact with HMRC in any other way. A general practice that files corporation tax returns and also runs payroll is not in that phase; it sits in whichever window matches its accounts position. Because it is the legal entity that registers rather than the service line, a group that runs payroll through a separate company needs to work out the window for that company on its own facts, not by reference to the main practice.
Could our own firm fail HMRC's registration conditions?
It is possible, and the conditions are worth reading rather than assuming. HMRC requires evidence that the business is supervised for anti-money laundering, such as a digital copy of the supervision certificate or a confirmation email. The business must not have relevant outstanding tax returns or unpaid tax unless covered by a payment plan, must not be subject to a decision refusing to interact, an anti-avoidance sanction or a stop notice, must not have relevant unspent convictions for fraud or tax offences, must not be formally insolvent, and must not be suspended or banned. The same conditions apply to named relevant individuals, who must also not be disqualified as directors in the UK or overseas.
Does registration change what an accountant may say to a client about financial advice?
No. Registration under the new rules concerns dealing with HMRC about tax, and it neither grants nor restricts anything in relation to regulated financial advice. The boundary is unchanged: observing that a client has surplus cash, no protection in place or a business sale approaching is observation, and recommending a particular pension, investment or product is a regulated activity that belongs with an FCA-authorised firm. What registration does change is the surrounding context. Both professions will sit inside the same HMRC registration regime by 31 March 2027, which makes the question of who is registered to do what for a given client easier to answer plainly.