HMRC's mandatory tax adviser registration under MMTAR closes on 18 August 2026. Accountancy firms must act now to avoid restrictions and penalties.
HMRC is urging eligible tax advisers to register under the new mandatory requirements before 18 August 2026, and for many accountancy firms and practitioners, time is running out.
The first phase of Modernising and Mandating Tax Adviser Registration (MMTAR) closes on 18 August 2026 for new tax advisers, or advisers interacting with HMRC without an agent services account (ASA), Self Assessment or Corporation Tax account.
The implications are serious. Tax advisers who miss their relevant registration deadline may face restrictions on their ability to interact with HMRC on behalf of clients. Where advisers continue acting without registering when instructed to stop, HMRC may apply sanctions including financial penalties. For any accountancy firm serving clients on self-assessment or corporation tax matters, this is a make-or-break compliance event.
Who Must Register?
The scope is broader than many practitioners initially realised.
Businesses interacting with HMRC in respect of another person's tax affairs must register as a tax adviser with HMRC. This captures not only specialist tax advisers but also general accountants, bookkeepers, and even conveyancers submitting stamp duty land tax (SDLT) returns on behalf of clients.
HMRC has been in discussions with the Law Society and real estate practitioners about how the new regime applies to firms whose only interaction with HMRC is submitting SDLT returns. Those discussions have reinforced the message: conveyancing firms are within scope. Filing an SDLT return on behalf of a client is sufficient, regardless of whether any tax advice is actually being provided.
Registration Windows and Phases
The scheme operates in phases.
MMTAR registration opened on 18 May 2026 and is being introduced in phases. Those who already have a Self Assessment or Corporation Tax account will need to register from 18 August 2026. Those who only provide third-party payroll services on behalf of clients and do not interact with HMRC in any other way will need to register from 18 November 2026. For each cohort, each cohort has a three-month window in which to apply for an agent services account (ASA) to register with HMRC.
Existing Agent Accounts
If your firm already holds an agent services account with HMRC, the pressure eases slightly.
If your firm already has an agent services account with HMRC, you don't need to register again. Instead, HMRC will contact your firm via your existing account to verify compliance with new conditions. However, HMRC will contact you through your account before the end of March 2027 to check that your firm meets the new registration conditions.
Grace Period on Application
One small mercy: firms will not be sanctioned if they have applied by 18 August but are still awaiting a decision on their registration. This means the deadline is to apply, not necessarily to be fully approved, provided you submit before the cut-off.
What This Means for Practice
The scheme is designed to raise professional standards.
HMRC's Director of Intermediaries said: These new requirements will help create a fairer, more transparent tax advice market, support those advisers who meet high standards, and give taxpayers greater confidence in the advice they receive.
The government is investing £36 million to help HMRC modernise tax adviser registration and make it simpler for advisers to interact with the tax system as part of its Plan for Growth.
In practical terms, if you advise clients on tax matters or submit returns to HMRC, you need to act now. Tax advisers who still need to register should act now to avoid disruption to their ability to support clients. Waiting until mid-August risks administrative chaos and the risk of falling into HMRC's enforcement spotlight.
For AOPA members, the message is clear: audit your client interactions, confirm the scope of your practice, check whether your firm has an existing ASA, and submit your registration application immediately if you have not already done so.