HMRC letter codes · Debt and payment letters (IDMS)
HMRC debt and payment letters, explained by code
Last reviewed 14 October 2026 · How Tax Letters checks its facts
Codes on HMRC's letters about tax that is owed or overdue, such as IDMS99, IDMS99P and OCA43. Each code page reports what HMRC, GOV.UK or a recognised body publishes about it, with the source and the day it was checked beside each fact.
Every Debt and payment letters (IDMS) code
- IDMS99 Amount overdueGOV.UK says IDMS99 is a letter HMRC sends to tell you that its records show a payment is overdue.
- IDMS99P Overdue Simple Assessment paymentGOV.UK says IDMS99P is a letter HMRC sends to tell you there is an overdue payment on your Simple Assessment tax bill.
- OCA43 Unpaid Income TaxGOV.UK lists OCA43 as an HMRC letter about unpaid Income Tax.
Information, not advice: these pages report what HMRC and GOV.UK say about each code. Your own letter comes first.
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.
Other series
- Self Assessment (SA)SA1, SA401, SA250, SA316, SA100 and more
- Corporation Tax (CT)CT600, CT603, CT41G, CT41G (Clubs), CT208 and more
- VATVAT1, VAT1A, VAT1B, VAT2, VAT7 and more
- National Insurance (CA)CA5603, CA5403, CA2490, CA4361, CA72A and more
- Child Benefit (CH)CH2, CH24A, CH105, CH995, CH459 and more
- Tax credits (TC)TC600, TC602, TC603D, TC603R, TC603UR and more
- One-to-many lettersNISPAR1MM, LCL2REG2MM, CompC1LPLRMM, CompC1LPLR(A)MM, HEIPO2MM and more
- Compliance check factsheets (CC/FS)The factsheets HMRC's compliance letters name.
- Every HMRC letter codeAll the series on one page.
- GOV.UK: Check if a letter you’ve received from HMRC is genuine (checked 14 Oct 2026)
- HMRC Debt Management and Banking Manual DMBM618330: Pre-enforcement: coding out: Self Assessment (SA) and Tax Credit (TC) overpayments: IDMS 99 letters (checked 14 Oct 2026)
- GOV.UK: Find out what to do if you owe money to HMRC (checked 14 Oct 2026)
- GOV.UK: Check if a QR code on 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: Pay your Self Assessment tax bill (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)