UK Tax Adviser Registration Deadline 18 August 2026: First Tranche Closes in 28 Days

Mandatory tax adviser registration (MMTAR) opened on 18 May 2026 under the Finance Act 2026. HMRC's 18 July 2026 press release warned that the first registration window closes on 18 August 2026 for new tax advisers and those interacting with HMRC on behalf of clients without an Agent Services Account (ASA), Self Assessment account, or Corporation Tax account.

ICAEW confirmed the timetable in regulations made on 13 July 2026 (SI 2026/807). The regime requires businesses paid to interact with HMRC on another person's tax affairs to register and meet minimum conditions — with sanctions including financial penalties and temporary or permanent bans for unregistered advisers who continue acting after being told to stop.

The first tranche covers new tax advisers and existing businesses that interact with HMRC for clients but lack an ASA, Self Assessment account, or Corporation Tax account. This includes some SDLT agents who have an SDLT online account but have not completed the tax adviser registration process.

Advisers who already hold an ASA are treated differently — they must apply during the window from 31 December 2026 to 31 March 2027, with registration taking effect from 18 August 2026 once approved. Do not assume an existing ASA exempts you from all MMTAR steps.

Businesses that only provide payroll services without an ASA have until 18 November 2026 to 17 February 2027 (third tranche). Financial services organisations without an ASA register from 31 December 2026 to 31 March 2027.

HMRC's fact sheet makes the order explicit: tax advisers must complete anti-money laundering supervision (AMLS) registration before applying for an ASA, because HMRC cannot accept ASA applications while AMLS status is pending.

Firms that started the ASA process without confirming AMLS approval may find their application stalled. The Law Society notes that advisers who apply by 18 August but await a decision will not be sanctioned — but continuing to interact with HMRC on behalf of clients after the deadline without having applied risks disruption.

Allow several weeks for both AMLS and ASA processing. With 28 days until the first window closes, advisers who have not started either process should act immediately.

Unregistered advisers will not be permitted to interact with HMRC on behalf of clients after their tranche deadline until they have applied and been accepted. HMRC says it will always give a reasonable opportunity to comply, but advisers who ignore stop notices face financial penalties and bans.

Businesses can continue interacting with HMRC during their three-month registration window and while HMRC considers their application. The critical date is applying before the window closes — not necessarily receiving approval by 18 August.

Second tranche advisers with a Self Assessment or Corporation Tax account but no ASA register from 18 August to 17 November 2026. Check GOV.UK guidance to confirm which tranche applies to your firm.

Confirm which MMTAR tranche your practice falls into. If you lack an ASA, Self Assessment account, and Corporation Tax account, the 18 August 2026 deadline applies — verify AMLS status and start the ASA application immediately.

Review all client-facing services including SDLT, VAT, and payroll to ensure every activity requiring registration is covered. Update client engagement letters to reflect the new regulatory framework.

FinnAccountings helps accountants and bookkeepers manage client compliance across Ireland and the UK — start a free trial to streamline tax workflows while your practice completes HMRC registration.