HMRC requires every invoice to have a unique, sequential identifying number - but it does not dictate a specific format. You can use plain numbers, a prefix plus numbers, or a year-based scheme, as long as the sequence never repeats and never has unexplained gaps.
Common, HMRC-compliant formats
Simple sequential: 1, 2, 3, 4...
Prefixed: INV-0001, INV-0002, INV-0003...
Year-based: 2026-001, 2026-002... (resetting the counter each year is fine, as long as the year prefix keeps each number genuinely unique)
Customer-prefixed: SMITH-001, JONES-001 (less common, but valid as long as the overall system is documented and consistent)
What actually breaks the rule
Reusing a number, even for a cancelled invoice - void it instead, do not delete it and reuse the number
Unexplained gaps in the sequence, which can look like missing invoices during an HMRC check
Running two separate, undocumented numbering systems at once (for example, different sequences on paper vs. digital invoices) with no way to reconcile them
What to do with a cancelled invoice
If you create an invoice by mistake or a job falls through, do not delete it and reuse its number - mark it as void or cancelled and keep the record. Your sequence should show 001, 002 (void), 003, not 001, 003 with 002 missing entirely; a visible gap with no explanation is exactly what an HMRC check looks for.
Why this matters more across multiple devices
A spreadsheet-based numbering system falls apart the moment you invoice from two devices - a laptop and a phone, say - since each one only knows the last number it personally used, and duplicates happen the moment you are not paying close attention. Invoito assigns each invoice number via a single counter tied to your account, not your device, so numbers stay sequential even when you create invoices from your phone one day and your laptop the next.
Does HMRC require a specific invoice numbering format?
No - HMRC requires every invoice number to be unique and sequential, but does not mandate a specific format. Plain numbers, prefixed numbers, or year-based schemes are all acceptable.
Can I reuse an invoice number if I cancel the invoice?
Can I restart my invoice numbering each year?