11 Days Left: First HMRC Tax Adviser Registration Window Closes 18 August

With 11 days left until 18 August 2026, the first registration window under Modernising and Mandating Tax Adviser Registration (MMTAR) is almost closed. HMRC’s GOV.UK guidance and Agent Update issue 145 both stress the same point: if you are paid to interact with HMRC about someone else’s tax affairs and you fall in the first tranche, you need an Agent Services Account (ASA) application in before the window ends.

MMTAR opened on 18 May 2026. The first tranche covers new tax advisers and advisers who interact with HMRC without an ASA, Self Assessment agent account, or Corporation Tax agent account. Advisers who already hold an ASA are not asked to re-register in this window — HMRC says it will contact them through the existing account if further information is needed.

Who must act by 18 August — and who waits

First window (18 May to 18 August 2026): new tax advisers, or those interacting with HMRC without an ASA, Self Assessment, or Corporation Tax account. That can include some Stamp Duty Land Tax agents who only hold an SDLT online account and have not completed tax adviser registration.

Second window (from 18 August 2026): advisers who already have a Self Assessment or Corporation Tax agent account but still lack an ASA. Agent Update 145 frames this as register from 18 August — use HMRC’s checker so you do not miss the wrong tranche.

Later windows cover sole third-party payroll providers (from 18 November 2026) and financial services organisations (from 31 December 2026). You can apply early even if your mandatory window opens later.

AMLS first, then ASA — avoid the sequencing trap

HMRC’s fact sheet is explicit: complete anti-money laundering supervision (AMLS) before you apply for an ASA. HMRC cannot accept ASA applications while AMLS status is still pending. Firms that reverse the order risk a stalled application in the final days of the window.

Have ready your business (and sole-trader personal) Government Gateway credentials, firm UTR and linked postcode, company and VAT numbers where relevant, AML supervisory body details and proof of supervision, and names of relevant people in the business. HMRC’s apply-for-ASA guidance lists the full pack.

During your open registration window — and while HMRC processes a timely application — you can generally continue to interact with HMRC for clients. The critical action by 18 August for tranche one is submitting the application, not necessarily receiving final approval that day.

What happens if you miss the window

Unregistered advisers are not permitted to keep interacting with HMRC on behalf of clients after their tranche deadline until they have applied and been accepted. HMRC says it will give a reasonable opportunity to comply, but ignoring stop notices can lead to financial penalties and temporary or permanent bans.

Joint ventures, investment structures, SPVs, and in-house group tax teams are generally outside scope where there is no genuine third-party advisory relationship — Agent Update 145 repeats that MMTAR targets paid third-party advice, not internal group arrangements. Confirm edge cases against HMRC’s scope guidance rather than assuming exemption.

How FinnAccountings supports review-ready client packs

While practices finish ASA and AMLS steps, organised client bookkeeping still has to move. FinnAccountings helps Ireland and UK accountants and business owners prepare bookkeeping and tax packs with Chartered Accountant insight before qualified sign-off. Start a free trial for review-ready drafts — we prepare packs; we do not register you as a tax adviser with HMRC or file client returns on your behalf.

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Related reading

See our earlier MMTAR overview for the full tranche timetable, and use the free tax calculator only for high-level Income Tax and National Insurance planning estimates.

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