HMRC has opened the second registration window under Modernising and Mandating Tax Adviser Registration, or MMTAR. As of 19 August 2026, the firms now in scope are advisers that already use Self Assessment or Corporation Tax accounts but still do not hold an agent services account (ASA), with the deadline set at 18 November 2026. (gov.uk) For accountancy firms and specialist tax firms, this is less about a distant policy announcement and more about keeping client work moving. A practice that continues acting through older HMRC routes without starting the required registration process is now working against a live compliance date, not a future housekeeping exercise. (gov.uk)

HMRC’s guidance says anyone paid to deal with the department about another person’s tax affairs is generally treated as a tax adviser unless an exemption applies. That is wider than some small firms may assume, because the test turns on what the business does rather than how it describes itself. (gov.uk) In practice, a small firm that files returns, sends claims or handles HMRC correspondence for clients will usually be within scope. By contrast, firms that only provide third-party payroll services do not need to register until 18 November 2026, and financial services organisations have a later start date of 31 December 2026. (gov.uk)

That staggered timetable matters because the payroll and financial services delays are tightly drawn, not broad carve-outs. HMRC’s internal manual says a business only falls into those later tranches if it solely provides those services; if it also carries out wider tax or accountancy work involving HMRC, it moves into the earlier timetable instead. (gov.uk) For mixed-service firms, that point is easy to miss. A bureau that runs payroll but also deals with Corporation Tax or Self Assessment queries for part of its client base should not assume it can wait until winter; on HMRC’s reading, the earlier window is the safer view. (gov.uk)

Early volumes suggest firms are already responding. HMRC says more than 4,000 applications were submitted and more than 2,000 accounts created in the first registration window, which focused on the smallest agent group, while the wider programme is backed by £36 million of government funding to modernise tax adviser services. (gov.uk) The official pitch is straightforward: one digital route, clearer checks and higher standards. For firms, the nearer-term question is time and process. Even where the application itself is free and online, partners and practice managers still need to review legal entities, existing HMRC credentials and whether they meet HMRC’s registration conditions. (gov.uk)

One detail likely to reassure firms is that client service is not meant to stop overnight. HMRC says advisers have three months from the start of their window to apply, can keep dealing with the department during that period and, once they have applied, can continue acting for clients while the registration is processed. (gov.uk) That said, the pressure should not be shrugged off. HMRC says advisers who fail to register when required may have their ability to act for clients restricted and could face sanctions or financial penalties. For client-facing firms, that makes MMTAR a business continuity issue as much as a regulatory one. (gov.uk)

Firms that already hold an ASA do not need to register again in this phase. HMRC says they will be moved to the new digital service by 31 March 2027 and contacted through their account if more information is needed, while advisers who missed the first window or are new to the market are being urged to apply as soon as possible. (gov.uk) That distinction matters for firms with several trading entities or a patchwork of older logins. One part of the business may already be covered, while another may still be relying on legacy access and now needs to check where responsibility sits before the second-window deadline closes. (gov.uk)

The timeline is now firmly set. MMTAR opened on 18 May 2026, the first window closed on 18 August 2026, the payroll-only tranche runs from 18 November 2026 to 18 February 2027, and the financial services tranche starts on 31 December 2026 before the wider move completes by 31 March 2027. (gov.uk) For small and mid-sized practices, the immediate task is simple enough: check whether the firm already has an ASA, confirm which legal entity is interacting with HMRC, and match current service lines against HMRC’s tranche rules. It is the sort of back-office change that can look minor until it starts affecting authorisations, response times and client confidence. (gov.uk)