HMRC letter codes · One-to-many letters
CompC1MRRMMLetter asking a company to check it paid the right amount of Corporation Tax
Published by its source on 31 July 2025. A date in the letter that has passed stays passed: what a passed date means.
Last reviewed 14 October 2026 · How Tax Letters checks its facts
In short
HMRC's letter CompC1MRRMM is headed “Check you’ve paid the right amount of Corporation Tax”.
“CompC1MRRMM … Check you’ve paid the right amount of Corporation Tax”
CIOT: copy of HMRC letter CompC1MRRMM, published on its page · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CompC1MRRMM. Each fact has its source and the day it was checked beside it.
When HMRC sends it
HMRC's letter CompC1MRRMM says HMRC has information that shows the company has associated companies but has not declared them when claiming Marginal Relief on a Company Tax Return.
“CompC1MRRMM … We have information that shows your company has associated companies, but hasn’t declared them when claiming Marginal Relief on a Company Tax Return.”
CIOT: copy of HMRC letter CompC1MRRMM, published on its page · checked 14 October 2026
CIOT said, on its page for HMRC's letter CompC1MRRMM, that HMRC had started a One to Many letter campaign directed at companies it had identified as having potentially not declared associated companies in their Corporation Tax returns and thus potentially having claimed higher marginal rate relief than they were entitled to.
“HMRC have advised us that they have recently commenced a One to Many letter campaign directed at companies that HMRC have identified as having potentially not declared associated companies in their Corporation Tax returns and thus having potentially claimed higher marginal rate relief than they are entitled to.”
CIOT: HMRC One to Many letter - Marginal Rate Relief · checked 14 October 2026
CIOT lists HMRC's letter CompC1MRRMM as “Letter 1 - Check you’ve paid the right amount of Corporation Tax”, and says that one letter contains a section titled 'What happens next'.
“Letter 1 - Check you’ve paid the right amount of Corporation Tax … one letter contains a section titled 'What happens next'”
CIOT: HMRC One to Many letter - Marginal Rate Relief · checked 14 October 2026
What it asks
HMRC's letter CompC1MRRMM asks the reader to check all Company Tax Returns for the accounting period that includes 1 April 2023, and all returns for periods after that date.
“CompC1MRRMM … Check all your Company Tax Returns for the accounting period that includes 1 April 2023 … You’ll also need to check all returns for periods after that date.”
CIOT: copy of HMRC letter CompC1MRRMM, published on its page · checked 14 October 2026
Free help and HMRC contacts
HMRC's letter CompC1MRRMM points to GOV.UK, search “Company Tax”, then “Making Changes”, to amend a return.
“CompC1MRRMM … To do this, go to GOV.UK, search ‘Company Tax’ and choose ‘Making Changes’.”
CIOT: copy of HMRC letter CompC1MRRMM, published on its page · checked 14 October 2026
HMRC's letter CompC1MRRMM points to GOV.UK, search “HMRC extra support”, for a reader whose health or personal circumstances make it difficult to deal with HMRC.
“CompC1MRRMM … If you have any health or personal circumstances that make it difficult for you to deal with us, please tell us. … For more information, go to GOV.UK and search ‘HMRC extra support’.”
CIOT: copy of HMRC letter CompC1MRRMM, published on its page · 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.
- CIOT: copy of HMRC letter CompC1MRRMM, published on its page (checked 14 Oct 2026)
- CIOT: HMRC One to Many letter - Marginal Rate Relief (checked 14 Oct 2026)
- GOV.UK: Get help from HMRC if you need extra support (checked 7 Oct 2026)
- GOV.UK: Appoint someone to deal with HMRC on your behalf (checked 7 Oct 2026)
- TaxAid: About us (checked 8 Oct 2026)
- Citizens Advice: About us (checked 7 Oct 2026)