A wholesaler's back office on a Saturday, a filing drawer open and old invoices fanned out.

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Can I claim interest on invoices my client already paid late?

The short answer

Usually. An invoice paid late still carries its interest at 8% over the Bank of England base rate4 and a fixed sum (£40, £70 or £100)8, and the Small Business Commissioner says both can be charged on any invoices which have been paid late3. Each invoice has its own six years12, counted from its due date. A fair late fee in your contract, or a deal to drop interest, can end the claim.

Mara, WolfX research desk
Written with AI, checked by a person · 8 October 2026

England and Wales only. WolfX is software, not a law firm.

The numbers

What a paid invoice can still carry.

6 yearsTo start a claim on each invoice, counted from its due date to be safe.12

£40–£100On top of interest, once for each late invoice, by its size.8 2

11.75%Yearly interest on invoices that went late in 2026. In 2025 it was 12.75% or 12.25%.4 5

What you can add

Six paid invoices of £3,000, each about 40 days late

Say a client owed you six £3,000 invoices, due on 31 March, 30 June, 30 September and 31 December 2025, then 31 March and 30 June 2026. Every one was paid 38 to 41 days late, and you didn't add interest at the time. Each invoice is its own debt and takes the rate for the half-year in which it went late: 12.75%, then 12.25% twice, then 11.75% three times5.

Due 31 March 2025, paid 9 May. Interest: £3,000 × 12.75% ÷ 365 × 39 days5£40.87
Due 30 June 2025, paid 8 August. Interest: £3,000 × 12.25% ÷ 365 × 39 days5£39.27
Due 30 September 2025, paid 10 November. Interest: £3,000 × 12.25% ÷ 365 × 41 days5£41.28
Due 31 December 2025, paid 9 February. Interest: £3,000 × 11.75% ÷ 365 × 40 days5£38.63
Due 31 March 2026, paid 11 May. Interest: £3,000 × 11.75% ÷ 365 × 41 days5£39.60
Due 30 June 2026, paid 7 August. Interest: £3,000 × 11.75% ÷ 365 × 38 days5£36.70
Fixed sums: 6 × £70.00 (invoices from £1,000 to £9,999.99)2£420.00
Owed on six invoices that are already paid£656.35

Interest stopped on the day each invoice was paid6, so nothing grows now. Most of the total, £420.00, is the six fixed sums. Counting from its due date, the first invoice has until 31 March 2031 for a claim to be started. The others follow, one each quarter.

The road ahead

Five steps. An evening's work, and no fee.

Step 1: Date every invoice that was paid late
List each invoice due in the last six years and paid after its due date, with both dates. Use the date you agreed6. If you agreed none, the 30 days begin with the later of delivery and the day they got your invoice6: that day is day 1, and day 30 is the due date.
If your terms ran past 60 days and the long wait was grossly unfair to you, interest can start after 60 days6 instead. Long payment terms have their own page.
Cost: FreeTime: 1 hour
Step 2: Check nothing stands in the way
Read the late-payment clause in your contract: a fair late fee there replaces statutory interest10. Then search old emails for any deal to drop interest or take less. Once the debt exists, you and the client are free to agree that9.
Cost: FreeTime: 30 minutes
Step 3: Work out each invoice on its own
Each invoice has one rate for all its days6: 8% plus the base rate on the 30 June or 31 December before it went late4. Interest is the invoice × rate ÷ 365 × days, from the day after the due date to the day paid.
Add the fixed sum2 once for each invoice; it is due the moment interest starts to run8.
Cost: FreeTime: 20 minutes
Step 4: Send one statement with a new invoice
Email the letter below with a new invoice1 for the total, and a pay-by date 14 days out. One letter that covers every invoice, with the working for each, is easier to pay than six reminders.
Cost: FreeTime: Same day
Step 5: Give them 14 days, then decide
Expect payment, a question about your working, or a reply that you accepted the delay at the time. If it goes quiet, the next step is a letter before action. To be safe, start any court claim within six years12 of each due date. The oldest invoice drops out first.
If they dispute it and the sum is large, a solicitor who does debt recovery can read your contract and emails.
Cost: FreeTime: 14 days

A letter you can copy

Letter adding interest to invoices already paid late

Send it once you have worked out each invoice and found no contract fee or old email that drops the interest.

New email

SubjectInterest and fixed sums owed on invoices paid late

Dear , of my invoices, due between and , were paid after their due dates. The interest and the fixed sums that the Late Payment of Commercial Debts (Interest) Act 1998 adds to a late payment were not claimed at the time. They are still owed. Below is each invoice with its due date, the date you paid, the days late, the rate, the interest and the fixed sum. Interest is simple interest at 8% above the Bank of England base rate on the 30 June or 31 December before the invoice went late, from the day after the due date to the day you paid. Interest in total: Fixed sums in total: Total due: A new invoice for is attached. Please pay it by to: If you believe any of this is wrong, please tell me before that date and say why. If I have not heard from you by then, my next step is a formal letter before action. Kind regards,

Tap a highlighted gap to see what goes in it.

Send it by email so it is dated, with the new invoice attached. Keep the statement, the old invoices and your bank statements together. If a smaller payment arrives marked 'in full and final settlement', reply in writing before you bank it, saying whether you accept it as that.

When not to bother

When it isn't worth it

Your contract set its own late fee, and it is a fair one. That fee replaces statutory interest10. Claim what the clause gives, and read the clause before you add anything up.
You agreed to drop it. Once the debt exists, you and the client are free to agree what happens to it9. Search your emails for 'no interest' and 'full and final settlement' first.
You took 90 days as normal and said nothing. Expect them to argue it. A course of dealing can count as a term11, and interest can be cut where your conduct makes that just7.
They pay on time now and you want the work. This letter can end a good relationship. If you mean to stay, put the terms you will hold them to in writing instead. Each month you wait, the oldest invoice moves closer to its six-year limit12.
An invoice fell due more than six years ago. In England and Wales a claim on a contract runs out after six years12. Leave that invoice out and claim the rest. Scotland and Northern Ireland set their own time limits, so check yours.

The Wolf's note

The letter is the easy part. The decision is whether you would start a claim on the oldest invoice if they ignore it. Make that choice first, and the pay-by date means something to whoever reads it.

Mr. Wolf · the AI inside WolfX

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