Is Your Tax Adviser Registered With HMRC? The 18 August Deadline
One month to check yours. Three minutes to do it.
The four-week deadline most clients have not heard about.
You pay someone to do your tax return. From 18 August 2026, every tax adviser who wants to keep filing on your behalf has to be on the HMRC register. If yours is not on it by then, your filing has a problem. Here is how to find out where yours stands.
You pay a tax adviser. For Self Assessment, for CGT, for IHT planning, for dividend work, for whatever the thing is that you do not have the time or the appetite to do yourself. You assume they are who they say they are. You assume HMRC knows who they are.
From 18 August 2026, HMRC is making that assumption explicit. Under a new scheme called MMTAR (Modernising and Mandating Tax Adviser Registration), any tax adviser who wants to keep interacting with HMRC on your behalf has to be on a register. If they are not on it by their phase deadline, they cannot file on your behalf, query your records, or speak to HMRC about your tax. They also face financial penalties if they carry on.
The deadline that matters most
The first window closes on Monday 18 August 2026. If your adviser is new, or has been filing your returns without an Agent Services Account (ASA), they have four weeks to get on the register or stop acting for clients.
What MMTAR actually is
HMRC has been bringing tax adviser regulation in piecemeal for years. MMTAR is the consolidation: a single, free, online register that replaces a patchwork of previous processes.
The principle is simple. If you are paid to talk to HMRC on someone else's behalf about their tax, you must register and be visible on the register. The same rule applies to a sole accountant doing a handful of Self Assessment clients, a partner at a Top 40 firm, and a freelance bookkeeper running payroll for small businesses.
| Window | Who must register |
|---|---|
| 18 May - 18 August 2026 | New advisers, or advisers without an ASA, Self Assessment or Corporation Tax account |
| 18 August - 18 November 2026 | Advisers with Self Assessment or Corporation Tax account but no ASA |
| 18 November 2026 - 18 February 2027 | Advisers who solely provide payroll services |
| 31 December 2026 - 31 March 2027 | Existing ASA holders plus financial services organisations |
Who HMRC considers a tax adviser
HMRC's definition is wider than most people expect. From the official guidance:
Anyone paid to interact with HMRC on behalf of someone else about their tax affairs is generally considered to be a tax adviser unless an exemption applies.
That covers accountants and tax advisers, but also bookkeepers doing HMRC correspondence on your behalf, Independent Financial Advisers submitting claims on your investment bonds, will writers or estate planners handling IHT paperwork with HMRC. If someone is doing tax-adjacent work for you and HMRC is on the other end, they almost certainly need to be on the register.
Why this matters for your filing
Most people who pay for tax advice are paying for one of three things:
- Annual Self Assessment, done by your accountant every January.
- Capital gains tax work, typically when you sell shares, a second property, or large chunks of an ISA-eligible investment.
- Inheritance tax or estate planning, done by an adviser or solicitor, often years in advance.
All three require someone to talk to HMRC on your behalf. If your adviser is suddenly unable to do that, you can end up:
- Missing a filing deadline because nothing was submitted.
- Holding CGT losses you cannot realise because the disposal cannot be processed.
- Paying the wrong amount of tax on a one-off event.
How to check (3 minutes)
Step 1: Ask them directly
Email your adviser this week with one question: "Have you registered for MMTAR, and what is your ASA reference?" A registered adviser will answer in minutes with their ASA number. An unregistered one will either say no or will not know what you are talking about. Both responses are useful information.
Step 2: Check the gov.uk tool
HMRC has published a checker at gov.uk/guidance/check-if-and-when-you-need-to-register-as-a-tax-adviser-with-hmrc. It is written for advisers, but the conditions are useful for clients to read.
Step 3: Check your agent authorisation
Sign in to your personal tax account at gov.uk/personal-tax-account and look under "Manage who can deal with HMRC for you." If your adviser is not there, that is the answer.
What to do if your adviser is not registered
You have three real options.
Wait
If their registration is genuinely days away and they can show you the application reference, give them a short window. No more than two weeks past 18 August.
File Self Assessment yourself for one year
It is more paperwork than you would like and less efficient than paying an adviser, but it is real and it works. Most people who do it once never want to do it again, which is fine. That is what advisers are for.
Switch advisers
Ask for a full handover pack (your UTR, agent codes, prior year returns, dividend vouchers, share disposals) and take it to a registered firm. Switching costs you a few hours. Not switching and discovering the problem in February costs you a £100 penalty plus the tax.
What HMRC is doing about non-registered advisers
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.
Translation: an unregistered adviser is not just operationally stuck. They can be fined, and continuing to act for clients after being told to stop is the kind of behaviour HMRC can pursue.
One action this week
- Email your adviser today: "HMRC's MMTAR deadline is 18 August. Are you registered, and what is your ASA number?"
- If they reply with a number, keep it with your tax records. If they do not reply within five working days, treat that as a no.
- If the answer is no, decide whether to wait, file yourself for one year, or switch. Do not leave this until January.
Sources
- HMRC press release: Tax advisers one month left to register under new rules (20 July 2026)
- gov.uk: Check if and when you need to register as a tax adviser
- gov.uk: Apply for an agent services account
This article is for UK residents. It is general information, not personal financial advice. If you are unsure how this affects your specific situation, talk to a registered tax adviser or HMRC directly.