Mandatory CT Filing Software 2026: HMRC's Digital Shift & What Accountants Need to Know Now

The End of an Era: HMRC/Companies House Joint Online Filing Service Closure

For many years, the HMRC and Companies House joint online filing service provided a convenient, free portal for UK companies to submit their annual accounts and Corporation Tax (CT600) returns simultaneously. This service, sometimes known as CATO (Company Accounts and Tax Online), simplified compliance for countless small and unrepresented businesses.

However, this era has officially concluded. The joint online filing service permanently closed on 31 March 2026. The decision to decommission the service stems from its inability to meet modern digital standards and align with recent changes in UK company law. HMRC and Companies House are moving towards more streamlined, digitally integrated services, encouraging the use of commercial software that offers enhanced features and better data integrity.

Mandatory CT Filing Software 2026: The New Landscape for Corporation Tax

The closure of the joint online service ushers in a significant, non-negotiable change for Corporation Tax filing. From 1 April 2026, all UK companies are legally required to submit their Corporation Tax returns (CT600 forms) and accompanying tax computations exclusively through commercial software.

This mandate applies to all UK limited companies, regardless of size or sector. It means that the free HMRC web form for Corporation Tax returns is no longer available. The accompanying company accounts must also be provided in the Inline eXtensible Business Reporting Language (iXBRL) format, which commercial software is designed to handle.

It is crucial to understand that this shift to mandatory commercial software for CT filing is separate from Making Tax Digital (MTD) for Corporation Tax. While HMRC's broader digital transformation agenda continues, MTD for Corporation Tax has not been mandated for 2026. However, the move to software-only CT filing aligns with the overarching goal of digitalising the tax system, aiming to reduce errors and improve efficiency.

Protecting Your Records: Urgent Action to Download Historical Corporation Tax Returns

One of the most immediate and critical actions arising from the service closure is the need to download and securely save historical Corporation Tax records. After 31 March 2026, you will no longer be able to access any previously filed CT600 returns, computations, or accounts submitted through the joint online service.

Failing to download these records could lead to significant data loss and compliance challenges. Historical returns are vital for:

HMRC Enquiries: HMRC can enquire into returns for up to six years, or longer if fraud is suspected. Access to past filings is essential for responding to such queries.
Audits and Reviews: Professional reviews, due diligence for company sales, or loan applications often require comprehensive historical financial and tax data.
New Accountants/Agents: If a client changes accountants, the new firm will require prior year comparatives and historical records to ensure continuity and accurate future filings.
Amendments: Should a past return require amendment, having the original submission readily available is necessary.

How to Download Your Historical CT Records: A Step-by-Step Guide

HMRC recommended that all users log in to the online filing service before 31 March 2026 to download copies of at least the last three years of filed returns. Many experts advise extending this to six years to cover HMRC's enquiry window, or even downloading all available records.

Here’s a general guide on how to proceed:

Log in to your HMRC online account: Access the Government Gateway using your business tax account dashboard.
Navigate to Corporation Tax: Select the relevant company if you manage multiple entities.
Access 'Track your submissions': This section typically lists your past filings.
Download each return: For each accounting period, select the option to view or download the return. You should aim to save the CT600 form, tax computations, iXBRL accounts, and any submission receipts.
Save as PDF/HTML: Most services allow you to save a copy in HTML or PDF format. Ensure you save these files to a secure, accessible location.
Verify Downloads: Open each downloaded file to confirm it displays correctly and contains all necessary information.

Action Item: Prioritise this task immediately if you haven't already. Create a robust digital backup strategy for these critical documents.

Choosing the Right Commercial Software for Corporation Tax Compliance

With the mandatory shift to commercial software, selecting the right solution is paramount for practice managers and compliance officers. The market offers a wide range of HMRC-recognised software, each with varying features and pricing models.

When evaluating options, consider the following key features:

HMRC Recognition: Ensure the software is officially recognised by HMRC for CT600 filing. HMRC provides a list of approved suppliers.
CT600 and Supplementary Pages: The software must support the full CT600 form and any supplementary pages relevant to your clients' needs (e.g., for R&D claims, group relief).
iXBRL Accounts Production: The ability to generate and submit iXBRL-tagged accounts is essential. Some software integrates this directly, while others may require separate tagging services.
Direct Filing to HMRC: Look for software that allows direct submission to HMRC via the Government Gateway, ensuring efficient and secure filing.
Validation Checks: Robust validation helps minimise errors before submission, reducing the risk of penalties and rejections.
User-Friendly Interface: An intuitive design and guided workflow can significantly improve efficiency, especially for practices managing multiple clients.
Data Integration: Software that integrates with your existing accounting platforms can streamline data transfer and reduce manual input.
Support and Updates: Reliable customer support and automatic updates to reflect changes in tax legislation are crucial for ongoing compliance.
Cost-Effectiveness: Evaluate pricing models (per-filing, subscription) against your practice's volume and specific requirements.

Preparing Your Practice for Seamless Digital CT Filing

The transition to mandatory commercial software for Corporation Tax filing requires proactive planning and execution within your accounting practice. This isn't just a technical upgrade; it's an opportunity to refine your compliance processes.

Client Communication: Inform your clients about the changes well in advance. Explain why the shift is happening and how it will impact their CT filing process. Outline the new procedures and any actions they may need to take, such as providing necessary data in a timely manner.
Software Procurement and Training: Select and implement your chosen commercial software. Provide comprehensive training for your team to ensure they are proficient in using the new system for CT600 submissions, iXBRL tagging, and record-keeping.
Process Review and Optimisation: Review your current Corporation Tax compliance workflows. Identify areas where the new software can automate tasks, improve data accuracy, and enhance efficiency. This is an excellent time to standardise procedures for all clients.
Data Migration Strategy: Plan how you will manage client data, especially historical records. Ensure seamless transfer of relevant information to your new software, if applicable, or establish clear protocols for accessing downloaded historical data.
Internal Compliance Checks: Update your internal compliance checklists and procedures to reflect the new software-only filing requirements. Ensure all staff understand the revised HMRC deadlines and submission protocols.

Beyond CT: Navigating the Broader Digital Tax Landscape

While this blog post focuses on the immediate changes to Corporation Tax filing, it's important for UK accountants and bookkeepers to view this within the broader context of HMRC's ongoing digitalisation agenda. The move to mandatory CT filing software is another step towards a fully digital tax system.

For instance, Making Tax Digital for Income Tax Self Assessment (MTD ITSA) is also set to commence from April 2026 for sole traders and landlords with qualifying income over £50,000. This will require digital record-keeping and quarterly submissions via MTD-compatible software. Furthermore, MTD for VAT is already mandatory for most VAT-registered businesses.

Embracing digital tools now, not just for Corporation Tax but across all compliance areas, will position your practice and your clients for future success. This includes robust registered office obligations management, diligent AML compliance, and efficient handling of all HMRC deadlines.

Actionable Checklist for Practice Managers

To ensure a smooth transition and maintain compliance, here’s an actionable checklist for practice managers:

Confirm Joint Service Closure: Understand that the HMRC/Companies House joint online filing service closed on 31 March 2026.
Mandatory Software Filing: Recognise that from 1 April 2026, commercial software is mandatory for all Corporation Tax (CT600) submissions to HMRC.
Download Historical Records: Immediately download and securely store at least the last 3-6 years of all client CT600 returns, computations, and iXBRL accounts filed via the old service. This must be done before 31 March 2026.
Select Commercial Software: Research and procure HMRC-recognised commercial software that meets your practice's and clients' needs for CT filing, including iXBRL accounts.
Train Your Team: Ensure all relevant staff are fully trained on the new software and updated filing procedures.
Communicate with Clients: Inform clients about the changes, the need for commercial software, and any adjustments to your service delivery.
Review Internal Processes: Update your internal compliance workflows and documentation to reflect the new digital requirements.
Stay Informed: Keep abreast of further HMRC digitalisation initiatives, including MTD ITSA, to ensure ongoing readiness.

Conclusion

The closure of the HMRC/Companies House joint online filing service and the subsequent mandate for mandatory CT filing software 2026 mark a pivotal moment in UK corporate tax compliance. This isn't a future possibility; it's a present reality that demands immediate attention from accounting and bookkeeping practices.

By proactively addressing the need to download historical records, carefully selecting appropriate commercial software, and preparing your team and clients, you can navigate this digital shift smoothly. Embracing these changes now will not only ensure compliance but also enhance efficiency, accuracy, and the overall digital resilience of your practice in the evolving UK tax landscape.