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My client refuses to pay the interest. What then?

The short answer

Still owed. Interest is a term the law writes into your contract1, and so is the fixed sum for late payment, from £40 to £1004, so saying no removes neither. You can claim both in court without their agreement, and the court fee for a claim of £300 or less is £359.

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 claim costs, and how long you have.

£35–£70Court fee to claim up to £1,000, by the size of the claim.9

14 daysHow long to give a company to reply, in a straightforward case.8

30 daysHow long to give a sole trader to reply before you claim.10

6 yearsHow long you have to start a claim, usually counted from the day payment fell due.11

What you can add

A £3,600 invoice, paid 45 days late

Say you invoiced another business £3,600, due on 14 July 2026. They paid on 28 August 2026, which is 45 days late, and now refuse the interest and the fixed sum. It went late in July 2026, so it carries 11.75% a year: 8% over the Bank Rate of 3.75% on 30 June 202612.

£52.15Interest: £3,600 × 11.75% ÷ 365 × 45 days12

£70.00Fixed sum (invoices from £1,000 to £9,999.99)4

£122.15Still owed to you on 28 August 2026

The invoice is paid, so the sum has stopped growing. A claim for £122.15 costs a £35 court fee, plus a £27 hearing fee if they defend it9. On the small claims track the court can order them to pay both back to you.

The road ahead

Four steps. A court fee only at the end.

Step 1: Read their reason first
Each reason has an answer in the Act. No interest clause in the contract? The law writes the interest in as a term1. Payment terms of 60 or 90 days? The agreed day holds up to 60 days2, and longer only if it isn't grossly unfair to you. For a public body it's 30 days.
Your invoice was late or wrong? The default 30 days run from when they had notice of the amount2, and your own conduct can cut the interest3. A dispute over the work is a different road.
Their terms set a lower rate or their own late fee? If you agreed it before the work and it is fair, it replaces the law's interest, so claim what the contract gives. If it is both too weak and unfair to rely on, the law's interest applies.
Cost: FreeTime: 10 minutes
Step 2: Reply once, in writing
Email the letter below and post a copy. It does the job of a letter before action: the basis, the sum, what you want8 and 14 days to reply. A sole trader gets 30 days10.
Don't skip it. A claimant who goes straight to court can be deprived of interest8. A sole trader also needs the protocol's Information Sheet, Reply Form and Financial Statement form10 with it. Don't charge interest on the interest.
Cost: A stampTime: Same day, then 14 days
Step 3: Or trade it, if speed matters more
Offer to drop all or part of the interest if the money reaches you by a date. Put it in writing, as a one-off, and treat it as final: once the debt exists, you are free to agree terms about it5. Not by phone, and not by a vague promise.
Cost: The interest you dropTime: Same day
Step 4: Claim it when the date passes
Your claim must say the interest is claimed under an enactment, and which one6, then the rate, the dates, the total and the daily rate, which is asked for because interest keeps running until they pay. If the invoice is still unpaid, add the interest to the claim for it.
The fee is £35 up to £300, £50 up to £500 and £70 up to £1,0009, counted on the claim plus interest. A hearing fee follows if they defend it. On the small claims track, court fees you paid can be ordered back to you7.
Cost: £35 to £70 court feeTime: An hour to start it

A letter you can copy

Reply to a refusal to pay the interest

Send it once they have said no, or ignored you, and the invoice itself has been paid.

New email

SubjectInvoice : interest and fixed sum still due

Dear , I am writing about invoice , for . It was due on and was paid on . You have said you will not pay the interest and the fixed sum that went with it. Under the Late Payment of Commercial Debts (Interest) Act 1998, the invoice carried: Interest at a year on for days (the day after the due date to the day it was paid): Fixed sum for late payment: The total is . Please pay it by , to the account named on the invoice. If it is not paid by then, I will start a claim in the County Court for the interest and the fixed sum, and ask the court to add its fee, without further notice. If you believe any of this is wrong, please tell me before that date and say why. Kind regards,

Tap a highlighted gap to see what goes in it.

Email it so it is dated, post a copy to a company's registered office, and keep both with the invoice. A phone call leaves no record. If they ignored you rather than said no, cut the sentence about what they said and the line answering their reason. If the invoice itself is still unpaid, ask for it in the same letter and make the interest run to today. Add what it grows by each day.

When not to bother

When it isn't worth it

The sum is about £100 and the client is a good one. Trade it, or let it go on purpose, in writing, as a one-off. Once you have agreed that, treat it as final5.
The refusal is part of a real dispute about the work. Settle that first, because a claim for the interest alone adds a second fight to the first. What a dispute does is on its own page.
The client is in financial trouble. A small claim for the interest is the wrong tool. Chase the invoice first, and put the interest into that claim.
The invoice fell due nearly six years ago. A claim on a contract runs out after six years11, so start it first and talk afterwards.
The client is in Scotland or Northern Ireland. These pages cover England and Wales only, and the court route there is different.

The Wolf's note

A no changes nothing in law. Before you reply, decide what this is worth to you: the sum, the fee, the client. Then write once, give a date, and on that day do what the letter says.

Mr. Wolf · the AI inside WolfX

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