The CT600 is the Company Tax Return for UK limited companies. The CT600L supplementary form is also required for claiming R&D tax relief.
A CT600 is the Company Tax Return that UK limited companies and organisations use to report their Corporation Tax position to HM Revenue and Customs (HMRC).
In this article:
CT600 – Company Tax Return
Who needs to submit a CT600 form?
You must submit a CT600 if HMRC issues your company with a notice to deliver a Company Tax Return. This applies even if your company:
- Made a loss
- Has no Corporation Tax to pay
- Is claiming reliefs such as R&D tax credits
Sole traders and partnerships do not submit a CT600, they file through Self Assessment instead.
What does the CT600 include?
When completing a CT600, your company calculates and reports:
- Its taxable profit or loss for Corporation Tax purposes
- Any Corporation Tax due
- Any reliefs or credits being claimed, such as R&D tax relief
The figures reported on the CT600 are based on tax rules and may differ from the profit or loss shown in your statutory accounts.
You can prepare and submit a CT600 yourself, or you can appoint an accountant or tax adviser to file it on your behalf.
CT600 filing and payment deadlines
- Filing deadline: 12 months after the end of the accounting period
- Payment deadline: Usually 9 months and 1 day after the end of the accounting period
Late submission can result in penalties, even if no Corporation Tax is due.
CT600L – Supplementary Form for R&D Tax Credit claims
If your company is claiming R&D tax relief, the CT600 alone is not sufficient. You will also need to submit a supplementary form. This is where the CT600L comes in.
What is the CT600L?
The CT600L is a supplementary form that must be submitted alongside the CT600 when a company makes an R&D tax credit claim.
It applies to companies claiming under:
While some high-level R&D figures still appear on the main CT600, the CT600L provides HMRC with significantly more detail about how the R&D tax credit has been calculated.
Why was the CT600L introduced?
Effective from 1 April 2021, the CT600L was introduced as part of HMRC’s wider effort to reduce error and fraud within the R&D tax relief scheme.
It was one of the first changes introduced to require more detailed financial information as part of an R&D claim. This approach was later reinforced by additional compliance measures, including the Additional Information Form (AIF) and the Claim Notification Form (CNF).
By requiring more detailed financial breakdowns, HMRC can:
- Better identify ineligible or high-risk claims
- Reduce delays caused by missing or unclear information
- Apply greater scrutiny to complex R&D claims
What information does the CT600L require?
The CT600L is more detailed than the R&D sections of the main CT600 and is used to report the financial calculation of an R&D tax relief claim.
It typically includes:
- Company and claim administration details
- A structured breakdown of how the R&D relief has been calculated
- Details of qualifying R&D expenditure used in the calculation
- How the resulting relief or credit is being used, for example:
- surrendered for a payable credit
- carried forward to future periods
- offset against Corporation Tax liabilities
The CT600L focuses on the tax computation and utilisation of the relief, rather than the technical justification for the R&D activities, which is provided separately through the Additional Information Form (AIF).
PAYE cap and CT600L exemptions
The CT600L includes sections that apply the PAYE and NIC cap to payable R&D tax credits, where relevant.
The amount of R&D tax credit that can be paid to a company is generally capped at:
£20,000 plus 300% of the company’s relevant PAYE and NIC liabilities
This cap is most relevant to loss-making companies claiming a payable credit, including those claiming under the merged R&D scheme or ERIS (Enhanced R&D Intensive Support).
PAYE/NIC cap exemption
The CT600L also allows companies to indicate whether they are claiming an exemption from the PAYE/NIC cap as part of their R&D tax calculation.
A company may be eligible for the exemption where:
- Its employees are actively creating, preparing to create, or managing relevant intellectual property
- No more than 15% of qualifying R&D expenditure relates to connected subcontractors or externally provided workers
Where an exemption is claimed, HM Revenue and Customs may expect supporting evidence to demonstrate the company’s role in developing and managing the relevant intellectual property.
Where to find official CT600 guidance
This page provides a high-level overview of the CT600 and how it links to R&D claims. For HMRC’s full official guidance, you can also refer to:
Company Tax Returns guidance on GOV.UK
https://www.gov.uk/company-tax-returns
