Authorizing a Tax Agent: A Professional’s Guide to 2026/27 UK Rules

For the 2026/27 tax year, authorizing a tax agent requires navigating a fundamentally transformed legislative landscape. Before an agent can establish a formal relationship with a client, the agent themselves must meet strict statutory prerequisites.

Section 223 of the Finance Act 2026 (FA 2026) actively prohibits unregistered tax advisers from interacting with HMRC on behalf of clients.  Interaction includes filing returns, contacting HMRC by phone or email, or sending messages through HMRC internet portals.  To operate, agents must register with HMRC and satisfy specific statutory conditions under section 227 of FA 2026, which mandate anti-money laundering (AML) supervision, absence of unspent relevant convictions, and no outstanding tax returns or overdue tax liabilities.  HMRC implemented this mandatory digital registration process beginning in May 2026, giving advisers a transitional window of at least three months to comply.

The Digital Handshake vs Paper Form 64-8

HMRC operates two primary mechanisms for authorizing a tax agent: the traditional paper route and the modern digital pathway.

Form 64-8 (Traditional Authorization)

Historically, taxpayers completed a physical Form 64-8 to inform HMRC that an agent represents them.  The taxpayer must specify which tax affairs they want the agent to handle, such as Self Assessment, Corporation Tax, or PAYE.  Form 64-8 sets the legal scope of the agency; by completing it, the taxpayer represents that their agent has the authority to deal with HMRC on the specified matters.  While still valid, HMRC prefers digital authorization to reduce processing times and manual errors.

The Digital Handshake (Agent Services Account)

For modern tax regimes, HMRC mandates a “digital handshake” via the Agent Services Account (ASA).  This process completely replaces the requirement for a paper 64-8 for specific digital services, including Capital Gains Tax on UK Property and Making Tax Digital (MTD).

“The digital handshake replaces the requirement for a 64-8 and is recorded on ETMP within the relationship category.”

To execute a digital handshake, the agent initiates the process within their ASA by generating a unique authorization link.  The agent emails this link to the client, who must sign into their own Government Gateway account within 21 days to accept or reject the authorization.

Table: Authorization Methods by Tax Service (2026/27)

MTD ITSA: Granular Authorisations and Multiple Agents

The introduction of Making Tax Digital for Income Tax Self Assessment (MTD ITSA) in the 2026/27 tax year fundamentally shifts how clients delegate tasks. Mandation applies from 6 April 2026 to sole traders and landlords with a qualifying income over £50,000.

Under MTD ITSA, authorizing a tax agent involves assigning specific roles, known as granular authorisations. Clients can appoint one “main agent” and multiple “supporting agents”.

• Main Agent: Holds full management rights, including making quarterly updates, viewing penalties, and finalising the end-of-year tax return.
• Supporting Agent: Operates with restricted access, focusing solely on keeping digital records and submitting the quarterly updates without finalising the annual tax position.

To smooth the transition, HMRC allows agents to automatically link their existing Self Assessment client authorisations directly into their ASA.  If an agent holds an existing SA authorization and adds it to their ASA, HMRC automatically treats them as the “main agent” for MTD ITSA purposes without requiring a new digital handshake.

Advanced Electronic Signatures for Repayments

When authorizing a tax agent to handle specific income tax repayment claims, practitioners must adhere to updated signature mandates. From 6 April 2025, HMRC requires Advanced Electronic Signatures (AES) when tax advisers submit nominations for repayments via P87, R40, and Marriage Allowance forms.  Taxpayers who are digitally excluded may still use traditional wet signatures, but agents processing volume claims must adapt their client onboarding workflows to capture AES-compliant authorizations.

Consider reviewing the specific evidence and documentation required by HMRC to demonstrate compliance with the Advanced Electronic Signature (AES) standard when processing high-volume R40 repayment claims.