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HMRC letter codes · Corporation Tax (CT)

CT600Company Tax Return

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

In short

GOV.UK says a Company Tax Return is made up of the form CT600, any supplementary pages, and company accounts and tax computations.

“A Company Tax Return is made up of: … the form CT600 … any supplementary pages … company accounts and tax computations”

GOV.UK: Company Tax Return obligations · checked 14 October 2026

Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CT600. Each fact has its source and the day it was checked beside it.

What it asks

For the CT600, GOV.UK says that if HMRC issues you a notice to deliver a Company Tax Return (CT603), you have a legal obligation to submit a return.

“If HMRC issues you a notice to deliver a Company Tax Return then you have a legal obligation to submit a return.”

GOV.UK: Company Tax Return obligations · checked 14 October 2026

Time limits and rules

For the CT600 return, GOV.UK's Company Tax Return obligations guide gives the deadline as 12 months after the end of the accounting period it covers. It also says that missing the deadline means a penalty for late filing, and that the deadline for paying the Corporation Tax bill is before the deadline for filing the return.

“The deadline for your return is 12 months after the end of the accounting period it covers. … If you miss the deadline you’ll have to pay a penalty for late filing. … The deadline for paying your Corporation Tax bill is before the deadline for filing your Company Tax Return.”

GOV.UK: Company Tax Return obligations · checked 14 October 2026

For a CT600 return called for by a CT603 notice to deliver, HMRC's CT211 notes say the filing date is the later of several dates, including 12 months from the end of the period for which the return is made and 3 months from the date the notice to deliver was served (see CT603).

“The return must be delivered to the officer who issued the notice to deliver by the filing date, which is the later of: … 12 months from the end of the period for which the return is made … 3 months from the date on which the notice to deliver was served”

GOV.UK: Corporation Tax: penalty determinations (CT211 Notes) · checked 14 October 2026

How to send it or apply

GOV.UK's CT600 page says the paper form can only be used to amend a return already submitted, or to file a paper Company Tax Return if you have a reasonable excuse or are filing in Welsh.

“You can only use this form to amend a return you’ve already submitted or to file a paper Company Tax Return if you: … have a reasonable excuse … are filing in Welsh”

GOV.UK: Company Tax Return (CT600) · checked 14 October 2026

For the CT600, GOV.UK's Company Tax Returns guide says to file the return online, and that a return can only be filed by post if you have a reasonable excuse or are filing in Welsh.

“You should file your return online. You can only file a return by post if you: … have a reasonable excuse … are filing in Welsh”

GOV.UK: Company Tax Returns · checked 14 October 2026

GOV.UK's CT600 page says a completed paper CT600 is sent to Corporation Tax Services, HMRC, BX9 1AX.

“After you’ve completed the return, send it to: Corporation Tax Services HMRC BX9 1AX”

GOV.UK: Company Tax Return (CT600) · checked 14 October 2026

Free help and HMRC contacts

For questions about the CT600, 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.