Invoice numbers: HMRC rules and numbering examples

Every invoice you send must have a unique identification number. If you are VAT registered, that number must also be sequential, drawn from one or more series, so each invoice can be identified on its own. Beyond that HMRC is flexible: you can use letters, prefixes, dates and separate series for different parts of the business.

What does HMRC say about invoice numbers?

GOV.UK’s guide to what an invoice must include lists “a unique identification number” as the first item on every invoice. This applies to sole traders, partnerships and limited companies, VAT registered or not.

For VAT invoices the rule is tighter. Regulation 14(1)(a) of the VAT Regulations 1995 requires “a sequential number based on one or more series which uniquely identifies the document”. HMRC repeats this in section 4.1 of VAT Notice 700/21, last updated 18 March 2024.

What counts as a valid invoice number?

A valid number is one that is unique and follows on from the one before it in its series. HMRC’s manual page VATREC5010 confirms these points:

• the number can be numbers only, or a combination of numbers and letters
• you can start at a higher number, such as 1135, as long as the series then runs consecutively
• you can run several sequences at once
• each division or business unit can use its own sequence
• a shared service centre can invoice for several group companies from one series

The older rule only asked for “an identifying number”. The sequential requirement came in through SI 2007/2085.

Invoice numbering examples

The formats below all work, provided each series runs in order without reusing a number.

If you use a customer code, keep a master sequence as well, or make sure the full reference is still unique across the business. Two invoices numbered 0012 for different customers would only be unique if the prefix is part of the number.

What if there is a gap in the sequence?

A gap is acceptable if you can account for it. VATREC5010 says breaks caused by cancelled or spoiled invoices are fine if you “retain the cancelled or spoiled invoice in your accounting records” or can explain the break.

In practice:

1. Do not delete a cancelled invoice. Mark it void and keep it.
2. If the invoice has already been sent, raise a credit note instead. See our guide to credit notes.
3. Never reuse a number, even after cancelling.
4. Write a short note for any gap you cannot match to a document, such as a software error.

Which rules apply to you?

The rules depend on whether you are VAT registered and how you invoice.

• If you are not VAT registered, each invoice needs a unique number. Sequential numbering is not written into the non-VAT rule, but it is the easiest way to prove uniqueness.
• If you are VAT registered, each VAT invoice needs a sequential number from one or more series, plus the other VAT invoice details. Our guide to VAT registration covers when you must register.
• If your customer self-bills you, they prepare the invoice. HMRC’s self-billing guidance says each self-billed invoice must show your name, address and VAT number and the statement “The VAT shown is your output tax due to HMRC”.
• If you issue a proforma invoice, it is not a VAT invoice and should not use your main series. See what a proforma invoice is.

What else must an invoice show?

The number is one of several required items. GOV.UK lists these for every invoice:

• a unique identification number
• your business name, address and contact details
• the customer’s name and address
• a clear description of what you are charging for
• the supply date and the invoice date
• the amounts charged, any VAT, and the total owed

Sole traders using a business name must also give their own name and an address for legal documents. Limited companies must show the full name on the certificate of incorporation, and if they name any director they must name all of them. Our guide to UK invoice requirements covers VAT invoice details in full.

How long must you keep invoices?

VAT Notice 700/21 says you must generally keep VAT records, including copies of the invoices you issue, for at least 6 years. Keep cancelled and spoiled invoices for the same period, because they explain gaps in your sequence.

What to do now

1. Pick one format and write it down so everyone who invoices uses it.
2. Let your accounting software number invoices automatically.
3. Use a separate prefix for credit notes and proforma invoices.
4. Keep every void invoice and note the reason.
5. If you charge late payment interest, quote the original invoice number; our late payment interest calculator works out the amount.

Frequently asked questions

Do invoice numbers have to be sequential in the UK?

For VAT invoices, yes: the VAT Regulations 1995 require a sequential number from one or more series. For businesses not registered for VAT, GOV.UK only requires a unique number, though sequential numbering is the simplest way to show that.

Can an invoice number include letters?

Yes. HMRC’s VATREC5010 says an invoice number can combine numbers and letters, as long as it forms part of a unique, sequential series.

Can I start my invoice numbers at 1000 instead of 1?

Yes. HMRC accepts a higher starting number, such as 1135, provided the numbers then run consecutively.

What happens if I skip an invoice number?

A gap is acceptable if you keep the cancelled or spoiled invoice or can explain the break. HMRC may ask about unexplained gaps during a VAT check.

Can I use different number series for different branches?

Yes. HMRC accepts several sequences running at once, including a separate series for each division or business unit.