HMRC letter codes · Debt and payment letters (IDMS)
IDMS99POverdue Simple Assessment payment
Last reviewed 14 October 2026 · How Tax Letters checks its facts
In short
GOV.UK says IDMS99P is a letter HMRC sends to tell you there is an overdue payment on your Simple Assessment tax bill.
“Letter IDMS99P … Simple Assessment … HMRC will contact you by letter to tell you there is an overdue payment on your Simple Assessment tax bill.”
GOV.UK: Check if a letter you’ve received from HMRC is genuine · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about IDMS99P. Each fact has its source and the day it was checked beside it.
What it asks
GOV.UK says the IDMS99P letter will tell you how to pay and what happens if you do not pay or call HMRC within 28 days of receiving it.
“Letter IDMS99P … The letter will tell you: … how to pay … what happens if you do not pay or call us within 28 days of receiving the letter”
GOV.UK: Check if a letter you’ve received from HMRC is genuine · checked 14 October 2026
If you cannot pay in full now, GOV.UK says the IDMS99P letter tells you to call HMRC on 0300 322 7835.
“Letter IDMS99P … if you cannot pay in full now, to call HMRC on 0300 322 7835”
GOV.UK: Check if a letter you’ve received from HMRC is genuine · checked 14 October 2026
GOV.UK says of letter IDMS99P: “You must not ignore the letter.”
“Letter IDMS99P … what happens if you do not pay or call us within 28 days of receiving the letter … You must not ignore the letter.”
GOV.UK: Check if a letter you’ve received from HMRC is genuine · checked 14 October 2026
Free help and HMRC contacts
GOV.UK’s page on paying a Simple Assessment tax bill, the bill IDMS99P is about, says you can get help if you cannot pay your tax bill on time.
“You can get help if you cannot pay your tax bill on time.”
GOV.UK: Pay your Simple Assessment tax bill · checked 14 October 2026
GOV.UK’s page on paying overdue tax in instalments, which the Simple Assessment payment page behind IDMS99P links to, says you may be able to set up a payment plan to pay your overdue tax bill in monthly instalments. It offers an online service to check if you are eligible and to set one up.
“You may be able to set up a payment plan to pay your overdue tax bill in monthly instalments. … Use this service to check if you’re eligible for and to set up a payment plan online.”
GOV.UK: If you cannot pay your tax bill on time · 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.
- GOV.UK: Check if a letter you’ve received from HMRC is genuine (checked 14 Oct 2026)
- GOV.UK: Pay your Simple Assessment tax bill (checked 14 Oct 2026)
- GOV.UK: If you cannot pay your tax bill on time (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)