CT600: Everything you need to know

The CT600 is the Corporation Tax Return form that is submitted to HMRC. It reports a company’s taxable profits and calculates the Corporation Tax due for an accounting period. 

A CT600 must be filed when HMRC issues a Notice to Deliver a Company Tax Return. In practice, most UK incorporated companies receive this notice. A return must be submitted unless HMRC formally withdraws it, including where a company is treated as dormant for Corporation Tax purposes.

This guide covers topics such as:

• What is a CT600?
• How does a CT600 link to R&D tax relief?
• Compliance areas that companies need to understand 

What is a CT600?

The Corporation Tax Return, also referred to as the CT600, is a formal document that is submitted to HMRC. It accompanies elements such as:

• Statutory accounts.
• A detailed tax computation.
• Any relevant supporting pages such as R&D, group relief, or loans to participants.

Who needs to file a CT600?

A company must file a CT600 where HMRC issues a Notice to Deliver and the notice has not been withdrawn. Even if a notice is not received, companies that are active in the UK are legally required to file a return. This will typically be 12 months after the end of their accounting period.

This generally applies to companies that:

• Are UK resident for Corporation Tax purposes, or
• Have UK taxable income or chargeable gains

A company must file even if it has no taxable total profits, unless HMRC confirms that no return is required.

When are the filing deadlines?

The deadline for filing a CT600 is very strict. The deadline for filing is 12 months after the end of the accounting period. 

The Corporation Tax payment deadline is usually 9 months and 1 day after the end of the accounting period, although large companies may be required to pay by installments. 

It is extremely important to remember that these two deadlines are separate obligations and should be monitored with caution. 

Late filing triggers automatic penalties and interest will accrue on any late payments. This is something that must be avoided to keep the R&D claiming process as smooth and compliant as possible. 

What is the structure of the CT600?

The main CT600 form includes:

1. Company information and accounting period details
2. Tax calculation summary
3. Losses and reliefs 
4. Groups and associated company information
5. Declarations by an authorised signatory 

Supplementary information may be required where it is relevant.

How does the CT600 link to R&D tax relief?

Where a company claims R&D tax relief, the CT600 records the financial effect of the enhanced deduction or expenditure credit calculated in the Corporation Tax computation. 

The applicable regime depends on the start date of the accounting period.

For accounting periods beginning on or after 1 April 2024, most companies fall within the Merged R&D Scheme. Loss-making SMEs that meet the R&D intensity threshold may instead qualify for Enhanced R&D Intensive Support (ERIS).

However, for accounting periods beginning before 1 April 2024, claims fall under the SME scheme or the Research and Development Expenditure Credit (RDEC) regime, subject to transitional rules.

To qualify, the claim must:

• Relate to qualifying activity that seeks to resolve scientific or technological uncertainty.
• Have incurred eligible expenditure.
• Be undertaken by competent professionals.
• Demonstrate an advance in science or technology.

An Additional Information Form (AIF) must be submitted before as the Company Tax Return. Without a valid AIF, the R&D claim will not be valid.

The CT600 does not contain the technical narrative. It records the financial outcome of the claim.

What are some common errors with CT600 submissions?

When it comes to compliance, some of the common issues are:

• Incorrectly adjusting accounting profit to calculate taxable total profits. 
• Failing to disclose associated companies, will affect tax thresholds. 
• Late or missing forms required to accompany the R&D claim 

Errors can lead to enquiries and penalties, meaning consistency and accuracy is key. HMRC has increased compliance activity when claiming for R&D, and CT600 filings are routinely assessed for errors, often being the initial trigger for a compliance check to begin with.

What does the CT600 declaration mean?

The Corporation Tax Return must be approved by a director or authorised adviser before submitting to HMRC. The declaration will confirm that the return is complete and as correct as possible. 

The person submitting the CT600 must understand what is being claimed in their name. According to the inaccuracy regime, penalties may arise where errors are deliberate or reasonable care has not been applied. 

The declaration of a CT600 will confirm that: 

• The return is complete and correct to the best of their knowledge.
• The company has complied with Corporation Tax requirements.

What is the practical process for amending a Corporation Tax return to revise R&D figures? 

In practice, amending a return to update R&D figures involves the following steps:

1. Prepare or restate the statutory accounts and corporation tax computation excluding any R&D adjustment. This establishes the baseline trading result and tax position before the R&D claim is applied. The underlying profit must be technically correct before layering in relief.
2. Recalculate the corporation tax computation to reflect the revised R&D claim. This includes applying the correct SME, RDEC or merged scheme treatment, updating enhanced deductions or expenditure credits, and revisiting loss utilisation, group relief, quarterly instalments, and deferred tax where relevant.
3. Then, prepare or update the R&D technical report and Additional Information Form (AIF). The project narrative, qualifying cost analysis, and cost breakdown must fully reconcile to the amended computation and accounts.
4. Amend the CT600 and any relevant supplementary pages to reflect the corrected R&D position. The amended return replaces the original submission and must be internally consistent across all schedules.
5. Ensure all amended documents, including the computation and accounts where required, are submitted in iXBRL format when filing online with HMRC.

It is important to keep in mind that paper filing is no longer accepted for most companies.

What final checks should be completed before a CT600 submission?

Before filing a CT600, a final check is very beneficial in the event of missing important information or there is anything that needs editing that could bring you issues from HMRC whether it is an enquiry or a penalty. Before making that submission, confirm that: 

• The accounting period matches the accounts and computation.
• Taxable total profits are accurate.
• R&D figures align with the documentation and Additional Information Form.
• Associated companies are correctly declared.
• The director or adviser has reviewed and approved the return.

Accuracy at the submission stage reduces risks of an HMRC enquiry and protects credibility with HMRC.

If you are seeking further advice regarding a R&D tax credit submission, please contact the Alexander Clifford team.