Tax Letters Independent · not HMRC Check your letter
Menu

App coming soon

HMRC letter codes · Corporation Tax (CT)

CT600LResearch and development

Last reviewed 14 October 2026 · How Tax Letters checks its facts

In short

GOV.UK says to use the CT600L form if you are claiming Research and Development Expenditure Credit or Research and Development SME Payable Tax Credit for small and medium-sized enterprises.

“Use this form if you’re claiming Research and Development Expenditure Credit or Research and Development SME Payable Tax Credit for small and medium-sized enterprises.”

GOV.UK: Corporation Tax: research and development (CT600L) · checked 14 October 2026

Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CT600L. Each fact has its source and the day it was checked beside it.

What it asks

GOV.UK's guidance on completing the CT600L says to complete the pages if your company is claiming both or one of: research and development (R&D) expenditure credit (RDEC), and R&D payable tax credit for small and medium sized enterprises (SME).

“Complete these supplementary pages if your company is claiming both or one of the following: … research and development (R&D) expenditure credit (RDEC) … R&D payable tax credit for small and medium sized enterprises (SME)”

GOV.UK: Completing the CT600L page for research and development · checked 14 October 2026

Time limits and rules

GOV.UK's guidance on completing the CT600L says that for accounting periods starting on or after 1 April 2024, the merged scheme RDEC and enhanced R&D intensive support (ERIS) will replace the old RDEC and SME schemes.

“For accounting periods starting on or after 1 April 2024, the merged scheme RDEC and enhanced R&D intensive support (ERIS) will replace the old RDEC and SME schemes.”

GOV.UK: Completing the CT600L page for research and development · checked 14 October 2026

How to send it or apply

GOV.UK's CT600L page says to check when you need to send an online Company Tax Return before you download the form and send it in the post.

“Before you download the form and send it in the post, check when you need to send an online Company Tax Return.”

GOV.UK: Corporation Tax: research and development (CT600L) · checked 14 October 2026

Free help and HMRC contacts

For questions about the CT600L, HMRC's Corporation Tax enquiries line (GOV.UK) is 0300 200 3410, open Monday to Friday, 8am to 6pm and closed on bank holidays. GOV.UK has a “Find out about call charges” link.

“Call HMRC for help with general Corporation Tax enquiries. … Telephone: 0300 200 3410 … Opening times: Monday to Friday: 8am to 6pm. Closed bank holidays … Find out about call charges”

GOV.UK: Corporation Tax: general enquiries · checked 14 October 2026

Free help first

These routes are described by GOV.UK, TaxAid and Citizens Advice themselves.

  • HMRC's extra supportHMRC offers extra support if your health condition or personal circumstances make it difficult to contact HMRC, for example a mental health condition, financial hardship or being treated in hospital.
  • Someone to deal with HMRC for youYou can authorise a friend, relative or voluntary organisation to talk to HMRC for you and help you fill in forms.
  • TaxAidTaxAid helps people with tax problems who cannot pay for advice. Tax Help for Older People merged with TaxAid in July 2025.
  • Citizens AdviceCitizens Advice gives confidential advice online, over the phone and in person, for free.