Release notes for legacy desktop version

UK - April 2025

ONESOURCE Corporate Tax v17.0

Release summary

At the beginning of April, HMRC updated the Corporation tax online service to include the latest business rules and validations (v1.993). ONESOURCE has also been updated to use the v1.993 business rules and validations for all FY 2015 and later submissions and offline validations in line with HMRC’s online service.
HMRC have also released updated versions of the following forms supporting those submissions:
  • CT600 (2025)
  • CT600F (2025) - Tonnage Tax
  • CT600L (2025) - Research and development
HMRC expect the latest version of the CT600 to be used from April, regardless of the period, as it supports the latest disclosures and business rules for submissions. Earlier versions of the CT600 can be used, but only provided no information is included in the computation that can only be returned on the latest version of the form.
We have made changes to the FY 2024 computation template packs and files to support the new 2025 CT600 forms, whilst continuing to support the existing 2024 CT600 forms. More details relating to the specific changes made are included below.
Be aware that this year will be the final year that we will be releasing new and updated template packs for the legacy ONESOURCE Corporate Tax software. Version 18.0 is due for release in July 2025, and will support accounting periods ending up to 31 March 2026. Accounting periods ending after that date will be supported on the web-based ONESOURCE Corporate Tax software.
These notes are structured to focus on the key functional areas of change, rather than specifying the changes made to each of the individual modules. Unless otherwise stated, changes have been made to all computation types.
A full list of the updated computation template packs included in this update can be found in the following Computation template packs released.

Updates - CT600 forms and supplementary forms

Area of change
Details and limitations
Support for the updated HMRC v1.993 online filing service, business rules, and validations
We updated ONESOURCE for the new v1.993 business rules and validations for online filing submissions, and offline validations.
These will be applied to all FY 2015 and later submissions and offline validations made following this release.
Support for the new CT600 (2025), CT600F (2025), and CT600L (2025)
We added support for the new CT600 form and CT600F and CT600L supplementary pages, relevant from the beginning of April 2025. This supports the disclosures and business rules for validations and submissions under v1.993 of the HMRC online service, however we continue to support the existing CT600 (2024) form together with the CT600F (2023) and CT600L (2022) supplementary pages where you are already using these.
Both CT600 options will be available from the
Tax calculation
 [A] and
Information for the return
 [P] sheet menus where the CT600 (2024) has not already been developed into the file. To replace the existing form with the CT600 (2025), simply erase the existing form.
All new computation template packs and existing files for FY 2024 have been updated to support the new forms, and a note is added to
W_FIXES
 to confirm when an existing file has been updated.
Changes in the CT600 (2025)
The main CT600 (2025) form changes are:
  • Schema updated to version 1.993 to cover the changes to the main CT600 form and CT600F.
  • A new return type for "Non-resident company" added to box 4 on the form.
  • A change to the purpose of box 650, so it must be completed where a creative claim is made by a company of any size in addition to an R&D claim made by a small company or SME.
  • A new check box 653 for claims made by R&D intensive SMEs.
  • A new check box 943 to indicate that R&D payable credit is to be paid to a nominee where certain conditions are applicable.
  • A new investment zone “East midlands" added for the CT600M.
We updated the
Information for the return
 [P] sheet to support the relevant options and flags above. All new computation template packs and existing files for FY 2024 have been updated as part of the support for the new CT600.
Updated CT600F (2025) form added
The main changes to the form are the inclusion of the new managed rate for tonnage tax and various text changes.
We updated the Tonnage tax computation sheet to support these changes, including a separate calculation of the “Daily profit - managed” where appropriate, and a separate table showing the relevant rates and limits for managed ships.
All new trade and multi-trade template packs and unsubmitted files for FY 2024 have been updated, and a note is added to W_FIXES to confirm when an existing file has been updated. As made clear in the note on W_FIXES, the new “Managed only” option and calculations will only be available from newly inserted rows on the Tonnage tax sheet where this already existed in the file.
Updated CT600L (2025) form added
The changes to the form are mainly text changes to incorporate the new enhanced R&D-intensive support (ERIS) and merged RDEC schemes.
New CT600P preparation
The new CT600P for creative industries was withdrawn by HMRC shortly before this release. However, we added and amended links from the Creative industry expenditure credit and Creative industries relief sheets to support the new boxes on the CT600 in respect of the creative expenditure credit and creative enhanced expenditure regimes:
  • Box 541 - Audio-Visual expenditure credit (AVEC)and Video Games expenditure credit (VGEC)
  • Box 614 - Audio-Visual expenditure credit and Video Games expenditure credit surrendered to this company
  • Box 663 – Creatives core expenditure - Box 665 – Creatives additional deduction
  • Box 886 - Payable Audio-Visual expenditure credit and Video Games expenditure credit.
We made the following changes to the Creative industry expenditure credit sheet to support the CT600P when it is released by HMRC (currently scheduled for April 2026):
  • Added a new data entry range for “Relevant global expenditure for this accounting period”.
  • The “Corporation tax liability for “step 1”” calculation now includes additional adjustments for SME/ERIS credits, RDEC amounts and income tax deducted applicable to the CT liability.
We likewise made changes to the Creative industry relief sheet to support the CT600P:
  • Added a new data entry range for “Core expenditure”, now required to complete the new CT600 box 663.
  • A new section to calculate the credit set-off against other liabilities.

Updates - other changes

Area of change
Details and limitations
Creative industries - new qualifying low-budget productions 53% rate
You can now claim the AVEC (low-budget film) tax credit of 53% on qualifying creative expenditure from 1 April 2024.
On the Creative industry expenditure credit sheet, choose "AVEC (low-budget film)" as the type of expenditure credit being claimed. The relevant credit rate has been included in the “Tax rates and expenditure percentages” section.
EOGPL changes from 1 November 2024
We updated the Energy (oil and gas) profits levy sheet to reflect the following changes effective 1 November 2024:
  • Increase in Energy profits levy rate from 35% to 38%.
  • Reduction in the decarbonisation investment allowance for qualifying expenditure from 80% to 66%.
  • Removal of the main 29% investment allowance for qualifying expenditure incurred on or after 1 November 2024.
Hire cap restriction offset - CT600 requirements
Following HMRC confirmation, we updated the Losses and allowances sheet, Calculation of tax liability sheet, and relevant CT600 boxes to ensure that boxes 275 and 285 of the CT600 are completed correctly when amounts are offset against current period non-ring fence profits from either current period or brought forward hire cap restriction.
CT600L box L75 – HMRC error and temporary workaround
The new merged RDEC rules introduced by Finance Act 2024 were incorporated into ONESOURCE Corporate Tax in our Q2 2024 release. At the time of the release, we were aware of an issue which we raised in the release notes as follows:
We have identified an issue with the CT600L for periods beginning on or after 1 April 2024 where the tax credit cap exception applies. Our understanding is that box L75 Total relevant expenditure on R&D workers’ PAYE and National Insurance contributions should be zero when the tax credit cap exception applies, and box L80 Step 3 restriction carried forward to next AP should also be zero as there is no restriction to carry forward. However, this scenario will trigger validation error 9746 which states that box L80 must be the maximum of box L70 less box L75 and zero. We have written to HMRC for guidance and await their response.
We received a response from HMRC on 27 March 2025 which acknowledges an error and sets out a temporary workaround until the problem can be resolved in the 2026 version of the CT600L. Unfortunately, due to this late notice, we were not able to include HMRC’s workaround for this release and the correction will be made in our next release. In the meantime, should you be affected by this scenario, please contact the Support team who will assist you in making the necessary changes to your computation.
HMRC’s updated guidance and full notes on the workaround can be found here. The key points are as follows:
"If you are claiming an exemption, please ensure that the amount entered in Box L75 equals the amount entered in Box L70, so the amount of Step 3 restriction carried forward to the next accounting period in Box L80 is zero.
You must also add a note in your computations or in a PDF attachment with your CT return, that you are claiming an exemption under section 112E CTA 2009. We will update the service in April 2026 to fix this issue."

Release development

Hosted customers
The update for ONESOURCE Corporate Tax v17.0 will be deployed by Thomson Reuters as part of our hosting services for ONESOURCE Corporate Tax users by the evening of Monday 21st April. We will minimise the downtime required for this update.
For further details relating to the scheduled maintenance, including the start time and end time of the maintenance, click here to access our system status page. ONESOURCE Corporate Tax is currently identified as Corporate Tax (legacy) [EMEA] and can be found under our “Direct Tax” applications.
If you would like to receive emails to notify of updates to our status page, you can subscribe through our status page. We recommend you use a modern browser such as Chrome, Firefox, or Edge to set your subscription preferences. More information about our new status page is available here.
Self-hosted enterprise customers
The update for ONESOURCE Corporate Tax v17.0 will be made available by Thomson Reuters via a secure download site during the week ending Friday 25th April. Installation instructions along with other relevant documentation will be included in the download file.
Non-hosted customers
The update for ONESOURCE Corporate Tax v17.0 will be made available on Tuesday 22nd April.
To be able to apply the update, you will need to have installed ONESOURCE Corporate Tax v17.0. If you have not yet installed v17.0, you can download the setup files from the Thomson Reuters download site. Installation instructions along with other relevant documentation will be included in the download file.
Once you have v17.0 installed, access the application, and select the Check for updates option from the “Help” menu. This will initiate the download of the relevant files. You should then close and restart the application and the updates will be applied.
If you are unable to install the updates via the OCT application, please contact our support team.

Computation pack - Computations, claims, and forms supported

Corporation tax, AIF, Investment trust and Life
Updated versions of the computation template packs covering periods ending in FY 2024 for the following company types:
  • Trading companies
  • Investment companies
  • Multi-trade companies
  • Oil ring-fence companies
  • Real estate investment trust (REIT) companies
  • Authorised investment funds (AIFs)
  • Investment trusts
  • Proprietary Life companies
  • Mutual Life companies
Which support, where required, the following Corporation tax return forms and supplementary pages:
  • CT600 v3 (2024) – Company tax return
  • CT600 v3 (2025) – Company tax return
  • CT600A v3 (2015) – Close company loans and arrangements to confer benefits on participators
  • CT600B v3 (2022) – Controlled foreign companies and foreign permanent establishment exemptions, hybrid, and other mismatches
  • CT600C v3 (2018) – Group and consortium relief
  • CT600D v3 (2015) - Insurance
  • CT600E v3 (2015) - Charities and Community Amateur Sports Club’s
  • CT600F v3 (2023) – Tonnage tax
  • CT600F v3 (2025) – Tonnage tax
  • CT600H v3 (2015) – Cross-border royalties
  • CT600I v3 (2019) – Supplementary charge in respect of ring fence trades
  • CT600J v3 (2015) – Disclosure of tax avoidance schemes
  • CT600K v3 (2017) – Restitution tax
  • CT600L v3 (2022) – Research and development
  • CT600L v3 (2025) – Research and development
  • CT600M v3 (2024) – Freeports and Investment Zones
  • CT600N v3 (2023) – Residential property developer tax

This article applies to:

  • PRODUCT: ONESOURCE Corporate Tax

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