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Agent Reference Numbers may be required when calling HMRC

2 September, 2026

We have received reports from members that HMRC staff on some phonelines are asking for Agent Reference Numbers (ARNs) as part of security checks before the agent can discuss their query with HMRC. Agents who cannot find or do not have an ARN have been unable to progress through the security checks. 

The ATT has urged HMRC to issue communications to agents highlighting this new requirement. We understand further details should be published via HMRC's Agent Update. 

Why have HMRC started asking for Agent Reference Numbers?

This additional security check on some HMRC phonelines is linked with HMRC's Modernising and Mandating Tax Agent Registration (MMTAR) program, despite not all phases of MMTAR being complete. We understand that HMRC's MMTAR guidance should be updated to cover their requirements regarding ARNs on telephone helplines shortly. 

For more information about MMTAR, please see our Mandatory Registration with HMRC - Frequently Asked Questions.

What is the Agent Reference Number, and where can agents find it? 

Pending guidance from HMRC, to avoid unnecessary delays and failed security checks, agents may benefit from locating their ARN before calling any HMRC helplines, and ensuring this number is shared with staff as necessary.  

The ARN is an 11-character reference that can be found under the welcome message on the front page of the Agent Services Account. It will start with four letters (which will include "ARN"), followed by seven digits - eg XARN1234567. 

For newly registered agents, there is normally a delay between registering with HMRC and an ARN being issued. We understand from HMRC that agents in this position should be able to use their registration application reference in place of the ARN.