Key takeaways
- The second window for mandatory tax adviser registration closes on 18 November 2026.
- It applies to advisers with Self Assessment or Corporation Tax accounts but no agent services account (ASA).
- Advisers who solely provide payroll services do not need to register before 18 November. Financial services organisations have until 31 December 2026.
- Registration is free and online. Advisers with a registration number can keep using HMRC's online services while the application is processed.
- HMRC may limit an adviser's ability to act for clients and can take enforcement action, including financial penalties, if they fail to register.
Tax advisers who have Self Assessment or Corporation Tax agent accounts but no agent services account must register with HMRC by 18 November 2026. This is the second phase of HMRC’s mandatory tax adviser registration. The first window closed on 18 August 2026, and HMRC is warning that advisers who do not register may be restricted from acting for clients.
What the registration is
HMRC calls the programme Modernising and Mandating Tax Adviser Registration, or MMTAR. It replaces a range of earlier processes with a single digital registration system. HMRC says the aim is to raise standards in the tax advice market, protect customers and support those who play by the rules.
Registration opened on 18 May 2026 and is being introduced in phases. More than 4,000 applications were submitted and over 2,000 accounts created in the first window, which targeted the smallest agent group. The government is investing £36 million to modernise HMRC’s tax adviser services.
Who must register by 18 November
| Group | Position |
|---|---|
| Advisers with Self Assessment or Corporation Tax accounts, without an agent services account (ASA) | Must register by 18 November 2026 |
| Advisers who solely provide professional payroll services | Do not need to register before 18 November |
| Financial services organisations | Do not need to register before 31 December 2026 |
| Advisers who missed the first window, and new entrants | Should register as soon as possible |
HMRC defines a tax adviser broadly. Anyone paid to interact with HMRC on behalf of someone else about their tax affairs is generally considered one. Eligible advisers must meet HMRC’s registration conditions to apply for an ASA.
What registration involves
Registration is free and online. HMRC provides step-by-step guidance and an interactive checker tool on GOV.UK to help advisers decide whether they need to register. An adviser who submits an application and receives a registration number can still engage with HMRC while the application is processed, and access to HMRC’s online services is not affected in the short term.
What happens if you do not register
HMRC says it may limit an adviser’s ability to act on behalf of clients if they fail to register when required. Advisers who continue to operate without completing registration could also face enforcement action, including financial penalties.
For practices, the practical risk is losing the ability to deal with HMRC for clients, which affects filing, queries and payment arrangements. HMRC’s help and support page for agents lists email updates, videos and webinars on the changes.
How it links to other adviser duties
Tax adviser registration is separate from anti-money laundering supervision and from Companies House authorisation as an Authorised Corporate Service Provider. Practices need to meet each requirement. See our guide to AML registration for accountants and the Companies House ID verification post.
Budget 2026 also says an intermediation roadmap for advisers is due in spring 2027 and that software providers connecting to HMRC will face registration and stronger standards. See HMRC compliance and enforcement.
Checklist for practices
- Check every adviser in the firm. Include anyone who deals with HMRC for clients, not just partners.
- Identify your agent accounts. Note which have Self Assessment or Corporation Tax access but no ASA.
- Use HMRC’s checker tool. It confirms whether you need to register.
- Register before 18 November. Keep the registration number.
- Diarise 31 December 2026 if you are a financial services organisation.
- Record the position in your compliance file.
Frequently asked questions
When is the tax adviser registration deadline?
18 November 2026 for advisers in the second registration window.
Do payroll-only advisers have to register by 18 November?
No. HMRC says advisers who solely provide professional payroll services do not need to register before 18 November.
Is registration free?
Yes. Registration is free and online.
What if I miss the deadline?
HMRC may limit your ability to act for clients and could take enforcement action, including financial penalties. Advisers who missed an earlier window should register as soon as possible.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance on 1 November 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.