72 Hours Left: HMRC Tax Adviser Registration Closes 18 August 2026

As of 15 August 2026, eligible tax advisers have about three calendar days left in HMRC’s first Modernising and Mandating Tax Adviser Registration (MMTAR) window. The tranche that opened on 18 May closes on 18 August 2026 for new tax advisers and those interacting with HMRC without an Agent Services Account (ASA), Self Assessment agent account, or Corporation Tax agent account.

HMRC’s 20 July press release warned that missing the relevant registration deadline can delay or disrupt client services. Where advisers keep acting without registering after being instructed to stop, HMRC may apply sanctions including financial penalties. Treat 18 August as a hard application date for tranche one — not a soft reminder.

Who must act before 18 August

Anyone paid to interact with HMRC on behalf of someone else about their tax affairs is generally a tax adviser unless an exemption applies. Tranche one covers new advisers and those without an ASA, Self Assessment, or Corporation Tax agent account. Advisers who already hold an ASA do not re-register in this window; HMRC will contact them through the ASA when more information is needed on the new digital system.

Solicitors and conveyancers who file or pay Stamp Duty Land Tax for clients are commonly in scope even when they do not give wider tax advice. Confirm your position with HMRC’s interactive checker and MTAR guidance before assuming an exemption.

What to finish in the next 72 hours

Complete AML supervision evidence before you start the ASA application — incomplete supervision is a common stall. Use HMRC’s step-by-step GOV.UK guidance and the interactive checker so you apply in the correct tranche rather than guessing.

Diary what happens after 18 August: advisers with a Self Assessment or Corporation Tax agent account but no ASA enter the second three-month window from 18 August to 18 November 2026. Payroll-only and financial-services tranches follow later. Do not confuse tranche-two opening with an extension of tranche one.

Keep MFA activation on a separate checklist. Opted-in agents still face the 19 August early activation day, and remaining agent IDs enter the 28 September–15 October automatic window — registration and MFA are related firm-risk items, not the same task.

How FinnAccountings helps while you close registration packs

Clean client ledgers make it easier to keep serving Ireland and UK businesses while firm registration work runs in parallel. FinnAccountings helps prepare bookkeeping and tax packs with Chartered Accountant insight before you or your ICAEW/ACCA or Chartered Accountant (Ireland) adviser files. Start a free trial for review-ready drafts — we prepare records; we do not register tax advisers with HMRC or file returns on your behalf.

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Related reading

For the final-day checklist as of 17 August, use the tranche-one close briefing before tomorrow’s deadline.

MMTAR tranche one: final day before 18 August →

Tranche-two firms should also read the 18 August opening briefing so ASA applications start on day one of the next window.

MMTAR tranche two opens 18 August 2026 →

Agents sequencing MFA beside registration can use the 48-hour activation checklist.

HMRC agent MFA: 48 hours to 19 August →