Every UK invoice needs a unique identification number, and a VAT invoice needs a sequential number from one or more series. You can run separate series for separate companies, brands or branches, but a number should never appear twice, so avoid restarting at 1 for each client or each year without a prefix.
In this article
The short version
- Every invoice needs a unique number; VAT invoices need a sequential number from one or more series.
- Separate series for separate companies or brands are allowed.
- Restarting at 1 each year or for each client creates duplicates unless the number carries a prefix.
- Fix a wrong or duplicate number with a credit note and a reissued invoice, never by editing it.
- Unique numbers make it easier to match payments to invoices.
What the law asks for
There are two levels, depending on whether you're issuing VAT invoices.
If you aren't VAT registered, GOV.UK's guidance asks for a unique identification number on every invoice. It doesn't have to run in order, but it does have to be different on every invoice you issue.
If you're VAT registered, a VAT invoice needs a sequential number based on one or more series that uniquely identifies the document. That comes from regulation 14 of the VAT Regulations 1995, and the "one or more series" part is useful: you're allowed several sequences side by side, such as one for each company or brand, as long as each one runs in order and never repeats itself.
You also have to keep a copy of every VAT invoice you issue, which is easier when each copy has a number nobody else in your ledger is using.
Which numbering schemes work
| Scheme | Example | Where it fits |
|---|---|---|
| One continuous sequence | 1041, 1042, 1043 | Most businesses, with the least to go wrong |
| A prefixed series per entity or brand | ABC-1041, XYZ-0207 | Two companies, two brands or two branches, each with its own sequence |
| Year prefix, continuous within the year | 2026-0148 | Fine as long as the year is part of the number, so 2026-0001 can't be confused with 2025-0001 |
| Client prefix | ACME-014 | Allowed if every number stays unique, but it makes ledger-wide checks harder, so a plain sequence with the client in a separate field is usually easier |
A couple of practical choices are worth making on day one. Many businesses start at 1000 or higher so their first invoices don't look like their first invoices, and there's no rule against it. Pick a width you won't outgrow, too, because four digits can run out sooner than you'd expect if you send a lot of small invoices.
If you invoice from more than one place, decide which system owns the sequence. An accounting package and a spreadsheet both handing out numbers from the same series will eventually issue the same number twice.
Credit notes need an identifying number of their own as well. Many businesses give them a separate series, such as CN-0001, so corrections don't take numbers out of the invoice sequence and are easy to spot in the ledger.
Gaps, duplicates and corrections
Gaps happen, usually when a draft is deleted or an invoice is cancelled before it goes out. HMRC can ask about a missing number, so it's sensible to keep cancelled invoices marked as void instead of deleting them, or to keep a short note of why a number was skipped.
A duplicate needs fixing properly. If two invoices have gone out with the same number, issue a credit note against one of them, reissue it on a fresh number and keep all three documents. The same route works for an invoice that went out with the wrong number: credit note, reissue, keep the record.
What you shouldn't do is edit the number on an invoice your customer already has. You'd end up holding a different version of the document from the one in their system, and their accounts team will be matching their payment to a number you no longer recognise.
If you change scheme, say from plain numbers to a year prefix, carry on from where the old sequence stopped or make sure the new format can't produce a number the old one already used.
Why numbering affects getting paid
Numbering looks like housekeeping, but it's what a payment gets matched on. When a customer pays, the reference they type is usually your invoice number, often shortened or mistyped. If that number is unique across your whole ledger, a near miss can still be matched. If two invoices share it, someone has to work out by hand which one has been paid, and occasionally gets it wrong and chases a customer who's already settled.
Payment links take most of that risk away, because the reference travels with the payment and nobody has to type it. With Adfin, every invoice goes out with its own payment link, and payments are matched back automatically, even when the reference is missing or only part of the amount is paid. Invoices and payments sync both ways with Xero and QuickBooks. Its collection agents only chase what's still unpaid, which depends on the match being right in the first place, and that's easier when every invoice has its own number.
Common questions
Does invoice numbering have to be sequential? For a VAT invoice, yes: the number must be sequential within one or more series. For other invoices it must be unique, and a sequence is the easiest way to keep it that way.
Can I use more than one invoice number series? Yes. The VAT rules allow one or more series, so separate sequences for separate companies, brands or branches are fine as long as each stays unique and in order.
Can I restart my invoice numbers each year? Only if the year is part of the number, so 2026-0001 can't be confused with 2025-0001. Restarting without a prefix creates duplicates.
Are gaps in invoice numbers a problem? Not usually, but HMRC can ask about them, so keep cancelled invoices marked as void and note why any number was skipped.
What number should my first invoice be? Any number you like. Many businesses start at 1000 or higher, and there's no legal minimum.
How do I fix an invoice that went out with the wrong number? Issue a credit note against it, reissue the invoice on a new number and keep both. Don't edit the number on an invoice your customer already has.
Sources
- GOV.UK - Invoicing and taking payment from customers: invoices, what they must include (accurate as of October 2026) https://www.gov.uk/invoicing-and-taking-payment-from-customers/invoices-what-they-must-include
- legislation.gov.uk - The Value Added Tax Regulations 1995, regulation 14 (accurate as of October 2026) https://www.legislation.gov.uk/uksi/1995/2518/regulation/14
- HMRC - Record keeping (VAT Notice 700/21) (accurate as of October 2026) https://www.gov.uk/guidance/record-keeping-for-vat-notice-70021
- Adfin - Pricing (accurate as of October 2026) https://www.adfin.com/pricing
