Changes and issues affecting the Corporation Tax online service
Source: gov.uk (2025-09-05)

How changes and issues may affect the way you should file your Company Tax Return.

Check the following Corporation Tax online service issues.

New requirements for Company Tax computation format

Most companies are required to include a computation when submitting their Company Tax Return. This computation:
- is typically generated by your accounting software
- includes the necessary eXtensible Business Reporting Language (XBRL) tagging
To improve consistency and provide clear expectations, HMRC is introducing standardisation for certain parts of the computation. The first two areas being addressed in this release are:
- accounts adjustments
- capital allowances
Additional sections will be introduced in future updates.

What you need to do

In most cases, no action is required. Your software provider should implement these changes automatically.

However, if you’d like to understand the new requirements in more detail, including the content, format, and XBRL tagging you can read about the HMRC prescriptions of computations (https://assets.publishing.service.gov.uk/media/6880ad58e44654d7423da589/CT-Computations-Format.pdf) (PDF, 517 KB, 59 pages).

The document outlines how HMRC expects these sections of the computation to be completed and includes:
- detailed tables
- explanatory notes
First-year allowance for zero-emission cars

As announced at Autumn Budget 2024, the 100% first-year allowance for qualifying expenditure on zero-emission cars has been extended to 31 March 2026 for Corporation Tax purposes. Find out more about capital allowances (https://www.gov.uk/government/publications/capital-allowances-extension-of-first-year-allowances-for-zero-emission-cars-and-electric-vehicle-charging-points).

We are aware that a small number of companies with an accounting period starting on or after 1 April 2025 and that intend to submit their tax return before April 2026, may be unable to report their claim for this allowance in boxes 726 and 751 of the CT600.

In April 2026, the Corporation Tax online service will be updated to support the extended period for claiming this first-year allowance. Before April 2026, if you do need to submit a claim for an accounting period:
- beginning before 1 April 2025, you can continue to do so using boxes 726 and 751
- beginning on or after 1 April 2025, report your claim in boxes 725 and 750 (other allowances and charges)
Research and Development expenditure credit (RDEC) claims

HMRC introduced the additional information form (https://www.gov.uk/guidance/submit-detailed-information-before-you-claim-research-and-development-rd-tax-relief) on 8 August 2023. You must submit this for all claims for Research and Development (R&D) tax relief or expenditure credit.

Research and Development expenditure credit (RDEC) claims which are exempt from the PAYE cap

The merged scheme R&D expenditure credit (RDEC) and enhanced R&D intensive support (ERIS) replaced the old RDEC and small and medium-sized enterprise (SME) schemes for accounting periods beginning on or after 1 April 2024.

We are aware that there is no option on the CT600L for customers to indicate that they are claiming exemption from the PAYE cap (https://www.gov.uk/guidance/research-and-development-rd-tax-relief-the-merged-scheme-and-enhanced-rd-intensive-support) under the merged scheme. A company can only claim to be exempt if they are creating or managing intellectual property under section CTA09/S1112E of the Corporation Tax Act 2009 (CTA) (https://www.legislation.gov.uk/ukpga/2009/4/section/1112E)

If you are claiming an exemption, you should make sure that the amount entered in box L75 equals the amount entered in box L70. This is so that the amount of step 3 restriction carried forward to the next accounting period in box L80 is ‘0’.

You must also add a note in your computations, or in a PDF attachment with your Corporation Tax return, that you are claiming an exemption under section 1112E CTA 2009.

We will update the service in April 2026 to fix this issue.

Creative industries

HMRC introduced the additional information form (https://www.gov.uk/guidance/support-your-claim-for-creative-industry-tax-reliefs) on 1 April 2024. You must submit this for all claims for creative industry tax reliefs or creative industry expenditure credits alongside the Company Tax Return. Use this new mandatory form to share supporting evidence, either before or on the same day the Company Tax Return is submitted.

HMRC planned to introduce new supplementary form CT600P for all creative industry claims for returns submitted from April 2025.

Due to a restriction within the updated HMRC Corporation Tax online service, we are aware that customers claiming one or more of the creative reliefs and credits will be unable to include the CT600P with their return.

We have therefore decided to temporarily postpone the requirement to include the CT600P in your return until April 2026. In the meantime, you should use this guidance.

Cultural Reliefs and Film, High End TV, Children’s TV, Animation and Video Games tax relief

Complete box 540 with the gross amount of your credit claim and box 885 with any balance of payable tax credit.

Audio-Visual Expenditure Credit (AVEC) and Video Games Expenditure Credit (VGEC)

If your software supports the new AVEC and VGEC boxes 541 and 886, complete box 541 with the gross amount of AVEC or VGEC and box 886 with the balance payable.

If your software does not support the new boxes, complete box 540 with the gross amount of AVEC or VGEC and box 885 with the balance payable.

In addition to the gross amount of AVEC or VGEC, there are other amounts you should include in box 540 or 541. For more information, read the guidance on completing your company tax return (https://www.gov.uk/guidance/the-company-tax-return-guide).

Read more about the new creative industry expenditure credits (https://www.gov.uk/guidance/corporation-tax-creative-industry-tax-reliefs).