Written by Hammad Rasool MAAT·AAT licensed accountant·Updated November 2026

What the IDMS99 is

GOV.UK lists the IDMS99 as a genuine HMRC letter about an amount that is overdue. It tells you a payment is late and that you must not ignore the letter. A version marked IDMS99P is used where the overdue amount is on a Simple Assessment tax bill.

Step 1: check that the debt is real and right

  • Check it is genuine: compare the letter with the list of genuine HMRC letters on GOV.UK, and sign in to your HMRC online account directly to see whether the same balance shows there.
  • Check your payments: a payment made with the wrong reference can sit unallocated while HMRC chases you for the same money.
  • Check the figure: if the bill is based on an HMRC estimate because a return was not filed, filing the return usually replaces the estimate with the true amount.

Step 2: if you can pay, pay

Pay using the reference on the letter and keep the confirmation. Interest runs daily on overdue tax, so every week of delay costs a little more.

Step 3: if you cannot pay in full, ask for a payment plan

If you cannot pay your tax bill in full, you may be able to set up a payment plan and pay in instalments. HMRC will check whether the plan is affordable for you. To set one up you will need:

  • the reference number for the tax you cannot pay, such as your Unique Taxpayer Reference
  • UK bank account details, and you must be authorised to set up a Direct Debit
  • details of your income and spending, or the company's if the debt is a company tax

If you cannot agree a plan with HMRC, it will ask you to pay the amount in full.

Key point: HMRC is far more flexible with people who make contact than with people who stay silent. Work out what you can truly afford each month before you call, and do not agree to a figure you will miss.
Example

Tom, a self employed plasterer, receives an IDMS99 for £3,850 after missing his January payment. He cannot pay in one go. We prepare a simple income and spending summary, and he agrees a monthly Direct Debit with HMRC. Interest continues, but the chasing letters stop and no further action is taken while he keeps to the plan.

What happens if you ignore it

The debt does not go away. HMRC adds interest and, depending on the tax, late payment penalties. It can then move to enforcement, which can include collecting through your tax code, using debt collection agencies, taking control of goods or court action. Each step adds cost and removes options.

If you think you do not owe it

Say so quickly and in writing, with the evidence. If the bill comes from a return you disagree with or a penalty you want to challenge, there are time limits, normally 30 days for an appeal. See our guide to appealing an HMRC penalty.

Common questions

Is an IDMS99 letter real?
GOV.UK lists IDMS99 as a genuine HMRC letter. Always confirm the balance by signing in to your HMRC account directly, not by using a link or number from a text or email.
Can I pay HMRC in instalments?
You may be able to set up a payment plan. HMRC checks that it is affordable, and you need your tax reference, bank details and details of your income and spending.
Will a payment plan stop the interest?
No. Interest continues on the unpaid amount, but keeping to an agreed plan stops enforcement action.
What is the IDMS99P?
It is the version of the letter sent when the overdue amount is on a Simple Assessment tax bill.

Checked against GOV.UK on 10 November 2026: Check if a letter you have received from HMRC is genuine, If you cannot pay your tax bill on time.

About the author

Hammad Rasool MAAT is an AAT licensed accountant based in Bolton. TaxTune Accountants looks after sole traders, landlords, contractors and limited companies in Bolton, Wigan, Bury, Salford, Oldham, Stockport and Manchester, and online across the UK.