The short answer: give every invoice a unique number in a sequence you can explain, and never reuse one. UK VAT invoices must carry “a sequential number based on one or more series which uniquely identifies the document” (HMRC), and EU VAT invoices a unique sequential number (European Commission). Formats such as INV-2026-014 meet that and stay easy to sort.
What the rules say
| Where | Invoice number rule | Source |
|---|---|---|
| UK, any invoice | “A unique identification number” | GOV.UK |
| UK, VAT invoice | A sequential number based on one or more series which uniquely identifies the document | VAT Notice 700, 16.3 |
| EU, VAT invoice | A unique sequential number identifying the invoice | European Commission |
| Canada, GST/HST | The CRA’s list of details a buyer needs to claim input tax credits does not include an invoice number | CRA |
| Australia, tax invoice | The ATO’s seven required details for a tax invoice do not include an invoice number | ATO |
| UAE, tax invoice | A sequential tax invoice number, or a unique number that identifies the invoice and its place in the sequence | VAT Executive Regulation, Article 59(1)(d) |
Where no number is required, use one anyway: it is how you, your client and an accountant find the invoice, match the payment and show that nothing is missing. Dutch invoices follow the EU rule; the details are in the Dutch invoice guide. For everything else a UAE tax invoice must show, see the UAE tax invoice generator.
Formats that work
| Format | Example | Suits |
|---|---|---|
| Simple sequence | 0001, 0002, 0003 | A first year with few invoices |
| Year and sequence | 2026-001, 2026-002 | Most small businesses; sorts by year |
| Prefix, year and sequence | INV-2026-014 | Businesses that also number quotes (Q-2026-014) and credit notes (CN-2026-003) |
| Series per shop or brand | LDN-2026-031, MCR-2026-012 | More than one location; each series stays sequential |
| Client code and sequence | ACME-007 | A few long-term clients, as long as each number is still unique |
HMRC’s wording, “one or more series”, is what allows a new series each year or one series per shop, provided no number repeats.
Starting, restarting and gaps
- Starting number: you don’t have to start at 1. Starting at 1001 is fine if you carry on from there.
- A new year: start a new series with the year in it (2027-001) rather than reusing 001.
- Pad the numbers: 007 rather than 7, so files sort in order.
- Gaps: if a number is skipped, note why (for example, voided before sending) so the sequence can be explained.
Cancelled or wrong invoices
Don’t delete an invoice you have sent or reuse its number. Issue a credit note with its own number that cancels all or part of it, then a new invoice if needed. For VAT, HMRC lists what a valid credit note shows, including its own number and date, the amount and VAT credited, and the number and date of the original invoice (VAT Notice 700, 18.2.3).
CREDIT NOTE CN-2026-003, 9 October 2026. Cancels invoice INV-2026-014 dated 2 October 2026 in full: £850.00 plus VAT £170.00.
Quoted the job first? Keep quotes in their own series and put the quote number on the invoice as a reference (turning a quote into an invoice). The invoice generator suggests the next number in your sequence and remembers it in your browser.
Sources checked 9 October 2026: GOV.UK, HMRC VAT Notice 700 (updated 25 June 2026), European Commission VAT invoicing rules, CRA input tax credits, ATO tax invoices (updated 18 September 2026). UAE row checked 17 October 2026 against the FTA’s consolidated Executive Regulation (issued 1 September 2026).