HMRC Free Filing Service Closure: Urgent Actions for Accountants

The Looming Deadline: HMRC Free Filing Service Closure on 31 March 2026

A significant change is on the horizon for UK accountants and the limited companies they serve. HM Revenue & Customs (HMRC) and Companies House are permanently closing their joint online filing service on 31 March 2026. This free service has long been a convenient option for many small businesses to submit their Corporation Tax returns (CT600s) and annual accounts simultaneously. From 1 April 2026, this option will no longer be available, making a proactive transition to commercial software mandatory for Corporation Tax compliance.

This post details the critical steps your practice must take immediately to prepare for the HMRC free filing service closure, focusing on data migration, software transition, and maintaining seamless compliance for your clients.

Why the Joint Online Filing Service is Closing

The decision to decommission the joint online service, sometimes referred to as the CATO portal, stems from a broader push towards modernising digital standards and aligning with recent changes in UK company law, notably the Economic Crime and Corporate Transparency Act. HMRC has stated that the existing platform no longer meets current digital requirements and that the commercial software market has evolved to better serve the needs of businesses and their agents.

The closure signifies HMRC's commitment to a fully digital tax system, moving away from its own free web-based submission tools towards a landscape where commercial software is the standard for filing. This aligns with broader digital transformation initiatives, including Making Tax Digital (MTD), even if not directly part of the MTD for Corporation Tax roadmap yet.

Critical Deadline: 31 March 2026 Demands Immediate Attention

The 31 March 2026 deadline is not merely a suggestion; it's a hard cut-off. After this date, you will lose the ability to use the free service for new submissions or to access any previously filed returns through it.

For Corporation Tax (CT600s): From 1 April 2026, all companies will be required to use HMRC-recognised commercial software to file their CT600s and accompanying computations. Paper filing will only be permitted in very narrow circumstances, such as having a 'reasonable excuse' or filing in Welsh.
For Companies House Accounts: While Companies House still offers web-filing and paper filing options for annual accounts post-March 2026, the clear direction is towards software-only submissions in the future. Many commercial CT software solutions integrate Companies House filing, offering a streamlined approach.

Ignoring this deadline carries significant risks, including late filing penalties that are set to double from 1 April 2026.

Essential Data Migration: Archiving Your HMRC Filing History

One of the most urgent actions for accountants is to secure historical client data. After 31 March 2026, access to past submissions made through the joint online service will be permanently removed.

Action Item: Download Historical Filings

You must download and securely save at least the last three years of your clients' Corporation Tax returns and accounts. HMRC recommends this as a minimum, but it's prudent to aim for six years, corresponding to HMRC's enquiry window, or even all available records.

Key documents to download include:

CT600 returns: The actual Corporation Tax return submitted.
Tax computations: Showing how the tax liability was calculated.
Filed accounts: The company accounts submitted with the return, often in iXBRL format.
Submission receipts/acknowledgements: Proof of filing date and acceptance.

How to Access and Download Your Clients' CT Filing History:

While specific steps may vary slightly, the general process involves:

Log in to the HMRC online service: Use your Government Gateway user ID and password.
Navigate to Corporation Tax: From your business tax account dashboard, select the relevant company if you manage multiple.
Access each accounting period: For each year you need, click on the accounting period to view the return details.
Download or save: Look for options like 'View return', 'Download', 'Save your return in HTML', or 'View PDF' to save the CT600, computations, and accounts.
Retrieve submission receipts: Download or print submission acknowledgements as PDFs for proof of filing.

Tip: Don't wait until the last minute. Address any login issues or missing returns well in advance of the deadline. Securely store these downloaded files, adhering to your AML compliance and data protection obligations.

Transitioning to Mandatory Commercial CT Software for Corporation Tax

The closure mandates the use of commercial software for all Corporation Tax filings from April 2026. This is a significant shift, especially for practices or clients who relied on the simplicity of the free service.

Action Item: Select and Implement New Software

Choosing the right HMRC CT600 software is paramount. Consider the following when evaluating commercial options:

HMRC Recognition: Ensure the software is officially recognised by HMRC for Corporation Tax submissions. HMRC provides a list of commercial software suppliers.
iXBRL Compliance: The software must be able to generate iXBRL-tagged tax computations and company accounts, which is the required digital format for HMRC.
Companies House Integration: Look for solutions that allow simultaneous filing of accounts to Companies House, streamlining the overall process.
Ease of Use & Features: Consider user-friendliness, automated calculations, validation checks, audit trails, and the ability to handle various company types (e.g., dormant, micro-entity, small).
Multi-Client Management: For practices, features like multi-client dashboards, bulk filing capabilities, and client communication tools are invaluable.
Support and Training: Evaluate the level of customer support and available training resources.

Many modern digital tools offer significant advantages over the old free service, including enhanced accuracy, reduced manual errors, and better integration with broader accounting ecosystems.

Developing a Proactive Transition Plan for Your Practice

A structured plan will ensure a smooth transition for your practice and clients, minimising disruption and maintaining compliance. This is more than just a software change; it's a procedural overhaul.

Action Item: Create a Detailed Transition Timeline

Identify Affected Clients: Review your client list to pinpoint all limited companies currently using the free HMRC/Companies House joint filing service.
Research & Select Software: Dedicate time to research and select suitable mandatory commercial CT software that meets your practice's and clients' needs. Start trials and demonstrations well in advance.
Data Migration Strategy: Beyond downloading historical data, plan how you will import client data into your new commercial software. Many solutions offer data import functionalities.
Staff Training: Ensure all relevant team members are thoroughly trained on the new software's functionality, filing procedures, and any changes to internal workflows.
Client Communication: Proactively inform affected clients about the change, explaining the reasons, the new filing process, and any implications for them. Offer support and guidance through the transition.
Test Submissions: Utilise test environments or file early returns with the new software to iron out any issues before critical deadlines.
Review Internal Processes: Update internal compliance checklists, such as those related to registered office obligations or HMRC deadlines, to reflect the new filing methodology.

Risks of Inaction and Ensuring Compliance Continuity

Failing to prepare for the Companies House joint filing end date and the mandatory shift to commercial software carries tangible risks for both your practice and your clients.

Late Filing Penalties: HMRC imposes fixed penalties for late Corporation Tax returns, even if no tax is owed. From 1 April 2026, these fixed penalties are set to double. For example, the initial £100 penalty will become £200, and an additional £100 for being three months late will become £200, escalating further for repeat offenders.
Tax-Geared Penalties: Beyond fixed penalties, companies may face tax-geared penalties (e.g., 10% of unpaid tax at six months late, another 10% at twelve months) if returns are significantly delayed and tax remains unpaid.
Loss of Historical Data: Without downloading, crucial historical filing data will be permanently inaccessible, complicating future compliance checks, financial reviews, or accountant handovers.
Disruption to Client Services: A last-minute scramble can lead to errors, delays, and a breakdown in client trust.
Reputational Damage: Consistent non-compliance can harm your practice's reputation and client relationships.

By taking a proactive approach to this Corporation Tax digital filing transition, you can safeguard your clients from penalties and ensure uninterrupted compliance.

Conclusion: Act Now for Seamless Corporation Tax Compliance

The closure of the HMRC and Companies House joint online filing service on 31 March 2026 is a definitive move towards fully digital Corporation Tax compliance. For UK accountants, this is a clear call to action. The urgency cannot be overstated – download historical client data, thoroughly research and implement suitable commercial CT software, and develop a robust transition plan.

Embrace this change as an opportunity to enhance your practice's efficiency and digital capabilities. By acting decisively now, you can ensure your clients remain compliant, avoid unnecessary penalties, and maintain confidence in your practice's ability to navigate the evolving tax landscape. Don't let this critical deadline catch you off guard.