Does my firm need to register under the new MTAR rules? A Guide for Solicitors and other Legal Professionals

If your firm ever contacts HMRC on behalf of a client, be that for filing a return, submitting SDLT forms, or discussing an estate’s tax position, you may need to register. MHA’s guide explores further.

The Finance Act 2026 introduced a new Mandatory Tax Adviser Registration (“MTAR”) regime. Under the new regime businesses that are paid to interact with HMRC on behalf of clients in relation to tax matters must register with HMRC and meet certain minimum standards. HMRC’s guidance is clear that the rules extend beyond traditional tax advisers and can apply to many legal professionals. For many solicitors, the key question is not whether they hold themselves out as tax advisers, but whether they interact with HMRC on a client’s tax affairs in the course of providing legal services.

All legal practices with any HMRC-facing activity should review the new rules and confirm whether MTAR registration is required. If so, they will also need to adhere to the conduct provisions included in the legislation.

Who Must Register?

Under the MTAR legislation, ‘tax advisers’ may not ‘interact’ with HMRC unless they are registered. Both terms are widely defined and will apply to many solicitors.

Interacting with HMRC includes:

• contacting HMRC by telephone, post or email;
• sending a message to HMRC through a website or internet portal;
• filing a return, claim, notice or other document with HMRC (whether electronically or otherwise);
• communicating with HMRC in any other way.

A ‘tax adviser’ is an individual or organisation that, in the course of a business carried on by it, assists other persons with their tax affairs. Assistance with tax affairs is provided if a person or organisation:

1. Advise another person in relation to tax;
2. Act or purport to act as an agent on behalf of the other person in relation to tax;
3. Provide assistance with any document that is likely to be relied on by HMRC to determine the other person’s tax position.

Putting both definitions together, many routine activities for solicitors could bring them within the scope of MTAR, such as:

• Acting as agent for a client.
• Preparing or submitting documents on which HMRC may rely.
• Filing returns, claims or elections.
• Making tax payments to HMRC on behalf of clients.
• Corresponding with HMRC regarding a client’s tax position.

You can read more in MHA’s guide here.