Release notes for legacy desktop version

UK - April 2026

ONESOURCE Corporate Tax v18.0

Release summary

For documentation and other product information, search the Customer Center.
Earlier this month HMRC updated the Corporation tax online service to include the latest business rules and validations (v1.994). ONESOURCE has also been updated to use the v1.994 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the following forms supporting those submissions, alongside the new supplementary page for creative industry reliefs, withdrawn by HMRC just before it was due to go live last year:
  • CT600 (2026)
  • CT600A (2026) - Close company loans and arrangements to confer benefits on participators
  • CT600E (2026) - Charities and Community Amateur Sports Clubs
  • CT600L (2026) - Research and development
  • CT600P (2026) – Creative industries
Changes have been made to the FY 2025 computation template packs and files to support submissions and the new 2026 CT600 form set, whilst continuing to support the existing 2025 CT600 form set. More details relating to the specific changes made are included below.
This content update provides updated template packs supporting computation templates for periods ending in Financial Year 2025 (FY 2025); updates to some computation template packs for periods ending in Financial Year 2024 (FY 2024) are also included. Full details of the updated template packs included in this update can be found at Computation and template packs updated.
Note that the Life computation templates have not been updated. These will be made available at a later date where required.
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.
HMRC require the new form CT600P to be submitted with all claims for creative industry reliefs and credits, which includes amendments to earlier years submissions even if the creative industry claim itself has not changed. Where this is the case, you will need to use the latest version of the CT600 form, together with the CT600P form.
A number of specific updates required to support submissions have been included, full details of which are included in the notes that follow:
  • Removal of the validation linking box 650 (SME R&D claims) to claims for creative reliefs
  • Changes to the completion of boxes 770 and 775 to include the majority of special rate assets
  • Support for claims for the new 40% first year allowances and the HMRC workaround for including in submissions
The legacy ONESOURCE Corporate Tax software only supports accounting periods ending up to, and including, 31 March 2026. The latest CT600 form set will continue to be made available each year, to allow support for late or amended submissions. Accounting periods ending after 31 March 2026 are supported on the cloud-based ONESOURCE Corporate Tax software, and the roll forward of files to migrate from legacy into cloud-based is supported.
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 form set and e-filing

Area of change
Details and limitations
Support for the updated HMRC v1.994 online filing service, business rules and validations
ONESOURCE CT has been updated for the new v1.994 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 the installation of this update.
Support for the new CT600 (2026), CT600A (2026), CT600E (2026), CT600L (2026) and CT600P (2026)
Support has been added for the new CT600 form together with the updated CT600A, CT600E, CT600L and the new CT600P supplementary pages, relevant from April 2026.
This supports the disclosures, business rules and validations for submissions under v1.994 of the HMRC online service. We continue to support the existing CT600 (2025) form however, together with the CT600A (2015), CT600E (2015) and CT600L (2025) 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 (2025) has already been developed into the file, you can replace this with the CT600 (2026) form by erasing the existing form.
Note that virtually all boxes that relate to creative reliefs or credits are now required to be linked from corresponding boxes on the new CT600P form. This means that any submissions containing creative reliefs or credits must use the CT600 (2026) form set and ensure the CT600P is completed, including all unsubmitted or amended forms whatever the period end.
All computation template packs and existing files for FY 2025 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.
Support for the new forms in earlier years
Support for the new form set has been added for periods ending between 1 April 2020 and 31 March 2025 via the "Latest CT600 form" develop option added to the
Tax calculation
 [A] and
Information for the return
 [P] sheets when files are first opened after the update has been installed.
As a reminder, if you create a new file for one of these earlier years, you should save, close and reopen the file so it is updated to provide access to the latest version of the CT600 form set.
No specific computation sheets are updated to support the use of the new form set for submissions, which can lead to occasional errors developing supplementary pages. In the majority of cases these will not impact the data being included on the relevant form, but the data should be checked carefully to confirm this is the case.
In some specific cases, including where a CT600P is now required, errors have been added to sheets to erase and re-develop them. The errors will usually be triggered when there is no CT600 form in the file, suggesting the file is being prepared for submission/re-submission, so it is recommended you check for errors in the file before developing the new form.
If there are any issues, then contact Customer Success for support and assistance.
Changes in the CT600 (2026)
The main CT600 (2026) form changes are:
  • The linked schema updated to version 1.994 to cover the changes to the main CT600 form, together with the changes to the CT600E and CT600L and the new CT600P.
  • The change introduced by HMRC last year to complete box 650 where a creative claim is made by a company of any size has been reversed. It is now only completed where an R&D claim has been made by a small company or SME.
  • HMRC have amended how box 770, and by extension box 775, are to be completed. HMRC now require expenditure on all special rate assets except cars to be included, rather than only long-life assets and integral features as previously required.
    • HMRC have agreed that any FY 2025 forms that have not yet been submitted should be amended, but submitted forms and earlier period files can be completed in accordance with the previous requirements.
  • A new investment zone of "Flintshire and Wrexham" has been added.
  • HMRC also added two further investment zones, unannounced, from April. We will add these in the next update.
The relevant sheets supporting the above changes have been updated. All computation template packs for FY 2025 have been updated. All existing files for FY 2025 have also been updated, with the exception of the box 770 change where only unsubmitted files have been changed.
Updated CT600A (2026) form added
The only changes made to the form by HMRC have been to amend the headings relevant to each part of the form, to confirm which section relevant return payments should be entered. No software changes have been required.
Updated CT600E (2026) form added
The main change to the form is the inclusion of details of "Legacy income", together with a list of legacies and details of the donors.
Whilst the majority of the CT600E is completed directly on the face of the form, we have added a new
Legacy donors details
 sheet to support the new page 4 of the form. The total will also feed through to the linked box E88 added to the main part of the form this year.
The new sheet can be developed from the
Information for the return
 [P] sheet, and is developed automatically when the CT600E (2026) is developed.
The trade, investment and multi-trade template packs and existing files for FY 2025 have been updated to allow the new sheet to be developed.
Updated CT600L (2026) form added
The main change to the form is the inclusion of new boxes L71 to L73A at "Step 3" requiring details relating to the tax credit cap and the PAYE and National Insurance contributions that relate to the calculations.
The section is only relevant where the period starts on or after 1 April 2024, and relief is only being claimed under the RDEC regime. Where relief is being claimed under the merged RDEC scheme and ERIS, updated HMRC guidance requires completion of the existing boxes L167 to L169A in the SME R&D and ERIS section instead.
To support this, a new flag labelled
R&D tax credit cap exception applies (CTA 2009, s1112E)
 has been added to the
Information for the return
 [P] sheet to replace the individual tax credit exception flags on the two R&D credit sheets.
We have also added a new range to capture the
Expenditure this period qualifying for RDEC
 to ensure that we can validate total R&D expenditure correctly.
Both the
Research and development expenditure credit
 and
R and D tax credit calculation
 sheets have been updated to enhance support for the merged scheme, and ensure that the updated CT600L form can be completed correctly.
Please note the validation issue highlighted by HMRC on their webpage Changes and issues affecting the Corporation Tax online service - GOV.UK. Adjustments have been made to ensure the workaround is followed when appropriate.
The trade and multi-trade template packs for FY 2024 and FY 2025 have been updated for the [P] sheet changes, and the changes to the two R&D credit sheets referenced above.
All existing files for FY 2024 and FY 2025 are updated for the [P] sheet changes, and an error added to any R&D credit sheets - triggered when there is no CT600 form in the file - advising that the relevant sheet is erased and re-developed to get the updated version.
A note is added to
W_FIXES
 to confirm when an existing file has been updated.
New CT600P preparation
The CT600P for creative industries was withdrawn by HMRC shortly before it was due to go live in April 2025, as a number of issues with the related validations had been identified. The
Creative industry expenditure credit
 and
Creative industries relief
 sheets had already been amended to support the original version of the CT600P, and these updated sheets were made available regardless of the withdrawal of the form.
The CT600P must now be completed for submissions for any period, if values exist in any of the following boxes on the CT600:
  • Box 540 – Creatives tax credit
  • Box 541 - Audio-Visual expenditure credit (AVEC) and Video Games expenditure credit (VGEC)
  • Box 885 - Payable creatives tax credit
  • Box 886 - Payable Audio-Visual expenditure credit and Video Games expenditure credit
Further changes have been made to the
Creative industry relief
 sheet, and the linked
Creative reliefs
 sheet used in multi-trade files, to support changes in the new version of the CT600P:
  • A new data entry range for
    UK core expenditure for CT600P
     has been added, to complete boxes P260B to P305B on page 6.
  • A new section to calculate the amount of
    Credit set-off against other liabilities – CT600P
    , to complete box P326 on page 6.
Please note the validation issue highlighted by HMRC on the webpage Changes and issues affecting the Corporation Tax online service - GOV.UK. Adjustments have been made to the
Creative industry relief
 sheet to accommodate the workaround suggested, and an error added to highlight when the workaround is required.
Further changes have also been made to the
Creative industry expenditure credit
 sheet to support the requirements of the new version of the CT600P:
  • A new data entry range for
    Relevant UK expenditure (included above) for this accounting period (CT600P)
    .
Changes have also been made to the
Creative industry AVEC and VGEC
 sheet, that provides the link between the
Creative industry expenditure credit
 sheet and the CT600P:
  • A new column for
    Relevant global expenditure for this accounting period that is UK expenditure
     linked to the new range above, to complete boxes P5B to P45B on page 2.
  • A new column for
    Additional credit for visual effects for this accounting period
    , to complete boxes P5E to P45E on page 2.
In some instances it is possible that a negative claim for visual effects credits can arise. Finance Act 2026, s33(5) confirms that where this occurs the expenditure credit being claimed should be reduced by the visual effects credit; this applies for periods beginning on or after 26 November 2025. Adjustments have been made to deal with this.
HMRC have provided no guidance on how this should be treated for periods beginning before that date. Please contact HMRC in this scenario, as submissions cannot be made that contain negative figures.
The trade and multi-trade template packs for FY 2024 and FY 2025 have been updated for the changes to the creative credit sheets.
For existing files for FY 2024 and FY 2025, an error is added to any creative credit sheets - triggered when there is no CT600 form in the file - advising that the relevant sheet is erased and re-developed to get the updated version.
A note is added to
W_FIXES
 to confirm when an existing file has been updated.

Update deployment

Hosted customers
The update for ONESOURCE Corporate Tax v18.0 will be deployed by Thomson Reuters as part of our hosting services for ONESOURCE Corporate Tax users over the weekend beginning Saturday 25th 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, go to 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 on this page.
Self-hosted enterprise customers
The update for ONESOURCE Corporate Tax v18.0 will be made available by Thomson Reuters via a secure download site during the week beginning Monday 27th April. Installation instructions along with other relevant documentation will be included in the download file.
Non-hosted customers
The update for ONESOURCE Corporate Tax v18.0 will be made available on Monday 27th April. 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.
If you are unable to install the updates via the OCT application, please contact the customer support team.

Computation packs updated

Corporation tax, AIF and Investment trust
Updated computation template packs for all computation types covering periods ending in FY 2025:
  • Trading companies
  • Investment companies
  • Multi-trade companies
  • Oil ring-fence companies
  • Real estate investment trust (REIT) companies
  • Authorised investment funds (AIFs)
  • Investment trusts
Updated versions of all corporation tax computation template packs above, except AIF and Investment trust, covering periods ending in FY 2024.
Supporting, where required, the following Corporation tax return forms and supplementary pages:
  • CT600 v3 (2026) or CT600 v3 (2025) – Company tax return
  • CT600A v3 (2026) or 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 (2026) or CT600E v3 (2015) - Charities and Community Amateur Sports Clubs
  • 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 (2026) or CT600L v3 (2025) – Research and development
  • CT600M v3 (2024) – Freeports and Investment Zones
  • CT600N v3 (2023) – Residential property developer tax
  • CT600P v3 (2026) – Creative industries
Standalone tax accounting
Updated computation template packs covering periods ending in FY 2025 for entity provisioning only calculations.

This article applies to:

  • Product: ONESOURCE Corporate Tax

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