HMRC letter codes · One-to-many letters
CompC11aESSMMLetter about sales that were not declared, with a final opportunity to make a disclosure
Published by its source on 19 March 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 CompC11aESSMM is headed “Please tell us now about sales you haven’t declared”. In this version the letter's example of a penalty uses £10,000 of tax.
“CompC11aESSMM … Please tell us now about sales you haven’t declared … if you owe £10,000 in tax, we can add a £10,000 penalty on top.”
CIOT: copy of HMRC letter CompC11aESSMM, published on its page · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CompC11aESSMM. Each fact has its source and the day it was checked beside it.
When HMRC sends it
HMRC's letter CompC11aESSMM says HMRC has information that shows the reader owes tax on undeclared sales, which may be because of misuse of a till system.
“CompC11aESSMM … We have information that shows you owe tax on undeclared sales. This may be because of misuse of your till system”
CIOT: copy of HMRC letter CompC11aESSMM, published on its page · checked 14 October 2026
CIOT said, on its page for HMRC's letter CompC11aESSMM, that HMRC had started a new One to Many letter campaign directed at individuals and businesses it suspected of using Electronic Sales Suppression (ESS) (till fraud) to under-declare their income.
“HMRC have advised us that they have recently commenced a new One to Many letter campaign directed at individuals and businesses suspected by HMRC of using Electronic Sales Suppression (ESS) (till fraud) to under-declare their income.”
CIOT: HMRC One to Many letter – Electronic Sales Suppression · checked 14 October 2026
Time limits and rules
CIOT said that HMRC's letters in this campaign, which include CompC11aESSMM, directed the recipient to tell HMRC about undeclared sales by making a disclosure within 30 days from the date of the letter using HMRC’s online ESS disclosure service on GOV.UK.
“The letter is directing the recipient to tell HMRC about their undeclared sales by making a disclosure within 30 days from the date of the letter using HMRC’s online ESS disclosure service on GOV.UK”
CIOT: HMRC One to Many letter – Electronic Sales Suppression · checked 14 October 2026
HMRC's letter CompC11aESSMM calls it the reader's final opportunity to make a complete, accurate and honest disclosure before HMRC calculates what it thinks is owed and charges penalties on top.
“CompC11aESSMM … This is your final opportunity to make a complete, accurate and honest disclosure before we calculate what we think you owe and charge penalties on top.”
CIOT: copy of HMRC letter CompC11aESSMM, published on its page · checked 14 October 2026
How to send it or apply
HMRC's letter CompC11aESSMM asks the reader to tell HMRC about any undeclared sales by making an online disclosure.
“CompC11aESSMM … Tell us about any undeclared sales by making an online disclosure.”
CIOT: copy of HMRC letter CompC11aESSMM, published on its page · checked 14 October 2026
Free help and HMRC contacts
HMRC's letter CompC11aESSMM points to GOV.UK, search “make a disclosure about misusing your till system”.
“CompC11aESSMM … Go to GOV.UK and search ‘make a disclosure about misusing your till system’.”
CIOT: copy of HMRC letter CompC11aESSMM, published on its page · checked 14 October 2026
HMRC's letter CompC11aESSMM points to GOV.UK, search “difficulties paying HMRC”, for a reader who might have difficulty paying tax owed after the disclosure.
“CompC11aESSMM … If you think you might have difficulty paying any tax you owe following your disclosure … Go to GOV.UK and search ‘difficulties paying HMRC’.”
CIOT: copy of HMRC letter CompC11aESSMM, 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 CompC11aESSMM, published on its page (checked 14 Oct 2026)
- CIOT: HMRC One to Many letter – Electronic Sales Suppression (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)