HMRC is consulting on: 

• the implementation of a standardised, fully tagged format (“full prescription”) for computations submitted as part of the company tax return; and
• mandatory online filing for amended company tax returns. 

The consultation closes on 2 June 2026.  

Full prescription: background 

Computations are submitted as part of the company tax return, alongside the CT600 and the company’s accounts. As companies can be quite diverse in size, and in the nature of the activities they carry on, some flexibility is allowed in the presentation and tagging (in eXtensible Business Reporting Language (XBRL)) of computations.  However, HMRC says that “too much divergence has evolved over time … resulting in significant variation in how essentially similar information is presented”. This can mean that “key information is omitted, presented ambiguously, or set out in formats that are difficult to interpret for both human and machine readers”.  

HMRC has been working on this issue with software developers and other stakeholders, including ICAEW, since 2024. In 2025, it published prescribed requirements for accounts adjustments and capital allowances. HMRC believes that this earlier engagement “provides a strong foundation for the proposals now being consulted on”.  

Full prescription: timeline 

HMRC recognises that full prescription may require significant updates to software products at a time when resources may be stretched by dealing with other changes, including the introduction of Making Tax Digital for income tax and the adoption of Pillar 2 reporting.

Therefore, HMRC proposes the following timeline for implementation of full prescription:

• Stage one: collaborative development and refinement of draft specifications (April to September 2026).
• Stage two: publication of the final full prescription (by the end of September 2026).
• Stage three: build and test period (October 2026 to September 2027).
• Stage four: live pilot period to allow HMRC to identify and resolve any issues, and to assess impacts on companies, etc, before enforcement begins (October 2027 to September 2028). During this period, all returns would be required to conform to the new format.

Full prescription: enforcement

HMRC’s preferred approach is voluntary compliance. However, it recognises the need for “proportionate enforcement measures to address persistent or deliberate non-compliance” and so is seeking views on the following options: 

• Replacing the current arrangements for indicating that software providers have met certain standards with an approved corporation tax software product list. Only products that pass HMRC testing and meet all prescribed formatting and tagging requirements would appear on the list. As is the case under the current arrangements, HMRC would not guarantee the tax accuracy of the software and products would be subject to periodic review.
• Blocking submissions that do not meet software standards. HMRC says that action would be taken “only in the most egregious cases” and would “include appropriate safeguards and prompt communication to minimise disruption for taxpayers”.
• Introducing penalties for non-compliant software providers where there are “deliberate or repeated breaches” of the rules. 

HMRC also proposes locking tags so that users cannot alter or remove them.

Amended tax returns

Currently, amendments to company tax returns may be submitted online or through the post, by writing or sending a paper tax return to HMRC. HMRC says that amendments made in written correspondence can:

• increase the risk of errors;
• result in delays in processing claims or refunds; and
• cause confusion as to whether a letter is an amendment or not. 

Therefore, it will become mandatory to file amendments to company returns online, subject to the specific exemptions set out in the consultation. The exemptions include amendments made during an enquiry and where the company is operated by individuals that are digitally excluded. 

HMRC is seeking views on the exemptions, including if any other exemptions should be considered, and whether mandation should commence later than 1 April 2027.