Contents
- What is changing on 18 November
- Where to find your firm's ARN
- Why HMRC paused the checks in August
- Which firms the check applies to
- The key dates in order
- How the check relates to mandatory registration
- What to do before 18 November
- Questions
General information for practice owners, accurate as at 12 October 2026. Figures and dates are as published by HMRC and reported by professional bodies. Rules and deadlines change, so confirm your position on GOV.UK and take your own advice.
From 18 November 2026, HMRC resumes asking firms for their agent reference number (ARN) when staff call the agent line or use webchat. The date falls on the same day the second tax adviser registration window closes. A practice that has delayed registration, or that has never told the team taking HMRC calls what the firm's ARN is, meets both issues on this date.
What is changing on 18 November
HMRC switched on ARN checks for the first time on 18 August 2026. Staff calling the agent line or using HMRC's webchat service could be asked to give the firm's ARN. HMRC used this to confirm the firm had registered for an Agent Services Account (ASA). HMRC paused the checks within a few weeks and said firms needed more time to prepare. The pause ends on 18 November 2026. From that date the checks apply to firms required to register with HMRC as tax advisers.
The check runs alongside the identity and client-authorisation steps HMRC already applies to every call. HMRC may also ask for a firm's ARN in other situations. A missing or unknown ARN can stall a call before the caller reaches the question they rang about.
Where to find your firm's ARN
The ARN is an 11-character code: a letter, followed by the letters ARN, followed by seven digits — for example XARN1234567. It sits on the Agent Services Account homepage, under the blue welcome message. Every firm with an ASA has one. HMRC introduced the ARN in 2017, and has required it on some claim forms, such as P87s and Marriage Allowance transfers paid to a nominee, since 2024. The checks from 18 November put this existing number to a new use: confirming a firm's status on a live call.
Firms with an ASA have an ARN already. The person who answers HMRC calls day to day does not always know it, or know where in the ASA account to find it. Writing it down in advance closes that gap.
Why HMRC paused the checks in August
HMRC has not published a detailed explanation for the pause beyond saying firms needed more time to prepare. The sequencing points to a practical problem. 18 August 2026 was also the date the second tax adviser registration window opened, for firms holding a Self Assessment or Corporation Tax account but no Agent Services Account. The check assumed a firm already had an ARN. A large group of firms was only starting the process of registering for one on that same date. Pausing the check gave that group its full three-month registration window to complete the process and find its ARN before being asked to produce it on a call to HMRC.
Which firms the check applies to
From 18 November, the check applies to firms required to register as tax advisers under HMRC's phased rollout. That covers any firm that deals with HMRC on a client's behalf for a fee: general practices, tax specialists, bookkeepers who liaise with HMRC for clients, payroll bureaux and overseas advisers acting for UK clients. In-house teams handling only their own employer's affairs sit outside the rule, along with a short list of specified exempt activities. The same exemptions apply to registration itself.
The identity, security and client-authorisation checks HMRC already runs on every call continue unchanged. A firm that already gives its UTR, postcode and authorisation details correctly on every call needs to add only one more detail: the ARN.
The key dates in order
These are the dates that matter, in sequence:
- 18 May 2026
- Registration window 1 opens — firms with no ASA, Self Assessment or Corporation Tax account
- 18 Aug 2026
- Registration window 2 opens; ARN checks on agent calls switched on, then quickly paused
- 18 Nov 2026
- Registration window 2 closes; window 3 opens (payroll-only firms); ARN checks resume
- 31 Mar 2027
- Final deadline — existing ASA holders and financial services firms
Illustrative — a call that goes wrong on 19 November
Take a six-partner general practice that registered for its ASA back in window 2, in September, and has not thought about the ARN since. On 19 November, the office manager calls the agent line about a client's PAYE coding query. Partway through the security questions, HMRC's adviser asks for the firm's ARN. It is not written down anywhere the office manager can see, and logging into the ASA from a desk without access to it means ending the call and ringing back. The query itself was a five-minute fix; finding the ARN mid-filing-season turns it into two calls and a lost morning slot. Writing the ARN on the internal HMRC-contact sheet now, next to the UTR and authorisation details already kept there, avoids this.
How the check relates to mandatory registration
The ARN check is the enforcement step behind the registration rules. Registration gives a firm its Agent Services Account and, with it, its ARN. The phone and webchat check confirms that the firm calling has completed that registration step. HMRC set the two dates to align: 18 August 2026 opened both the ARN check and the second registration window on the same day, and 18 November 2026 closes that window and resumes the check on the same day too. We set out the full registration timetable, including which window applies to which firm, in our article on HMRC's mandatory tax adviser registration.
What to do before 18 November
- Find the firm's ARN now. Log into the Agent Services Account and note the number from the welcome box.
- Put it where the team can see it. Add it to the sheet or shared note that already holds the firm's UTR and authorisation details for HMRC calls.
- Brief everyone who rings HMRC. Office managers, bookkeepers and junior staff are often the ones on the phone when the question comes up.
- Confirm the firm's registration status separately. Check it against the registration timetable to see which window applies to your firm. The ARN check is one part of a larger set of registration deadlines.
- If capacity is already stretched by MTD sign-ups and the registration deadline landing in the same quarter, our guide on outsourcing your back office covers freeing up a partner's time for this kind of compliance admin.
