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Tax adviser registration deadline: what small firms should check before 18 August

Pen-and-ink illustration of a UK tax adviser reviewing a registration checklist, with a small Union Jack as the only coloured element

Tax advisers who are new to dealing with HMRC, or who currently interact with it without certain online accounts, have until 18 August 2026 to register under a new mandatory system. The immediate duty falls on advisers, but small businesses that rely on an accountant or tax agent should also pay attention: a missed deadline could interrupt the help they receive with tax administration.

What is changing?

HM Revenue & Customs is introducing Modernising and Mandating Tax Adviser Registration, known as MMTAR. It is a single online registration system intended to replace several previous processes and make adviser interactions with HMRC more consistent.

The first registration window opened on 18 May and closes on 18 August 2026. It covers new tax advisers and advisers who interact with HMRC without an agent services account, Self Assessment account or Corporation Tax account. Registration is free and completed online.

Someone who is paid to interact with HMRC on another person’s behalf about their tax affairs will generally count as a tax adviser, unless an exemption applies. Advisers who already have an agent services account do not need to register again during this first phase; HMRC says it will contact them directly if more information is required.

Why the deadline matters to small businesses

Many small firms depend on an external accountant, bookkeeper or specialist adviser to handle submissions, queries and correspondence. HMRC warns that an adviser who should register but does not may have their ability to interact with the department restricted. Continuing to act after being instructed to stop can also lead to sanctions, including financial penalties.

For a client business, the practical risk is disruption at an awkward moment: an agent may be unable to deal with HMRC as expected, potentially slowing a filing, query or correction. That does not transfer the adviser’s registration obligation to the client, but business owners remain responsible for making sure their own tax affairs are handled on time.

What advisers should do now

Advisers in the first phase should use HMRC’s online guidance and eligibility checker now rather than waiting for the final days. They should confirm whether the requirement applies, collect the information needed for the application and allow time to resolve any account-access problems.

Eligible advisers must meet HMRC’s registration conditions before applying for an agent services account. Firms should also review who within their organisation communicates with HMRC, especially where tax work is only one part of a wider bookkeeping, payroll or consultancy service.

What SME clients can check

Small-business owners do not need to audit their accountant, but a short, sensible conversation can reduce uncertainty. Ask whether your adviser is affected by the new registration rules and, if so, whether they expect to complete the process before their relevant deadline. Confirm who will handle urgent HMRC correspondence if access is temporarily delayed.

It is also worth keeping your own records and account access in order. Make sure HMRC letters reach the right person, retain copies of submissions and authorisations, and avoid assuming that appointing an agent removes every responsibility from the business. Never share personal sign-in credentials simply to work around an adviser-access problem.

Later phases of the rollout

The August date is not the only deadline. HMRC’s phased timetable says advisers with a Self Assessment or Corporation Tax account but no agent services account will have from 18 August to 18 November 2026. Advisers who solely provide payroll services are scheduled for 18 November 2026 to 18 February 2027. Existing agent-services-account holders and certain financial services organisations follow from 31 December 2026 to 31 March 2027.

The key takeaway is to identify the correct phase, not assume the first deadline applies to everyone. For advisers covered now, however, 18 August is close enough to warrant immediate action. For their SME clients, a quick check can help prevent an avoidable break in support.

Source

HMRC: Tax advisers — one month left to register under new rules

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