An invoice number looks like a detail, but it is one of the few things HMRC specifically checks on a VAT invoice, and a broken sequence is one of the first things an inspector notices. A gap suggests an invoice that was issued and never recorded, which means sales and VAT may be missing.
What the rules say
For VAT-registered businesses, HMRC’s VAT Notice 700, section 16.3 says a full VAT invoice must show a sequential number based on one or more series which uniquely identifies the document, alongside the other required details. Our VAT invoice requirements guide walks through each item.
Businesses that are not VAT registered have no specific numbering rule, but a unique number on every invoice is still the only practical way to match payments, answer customer queries and show HMRC that your recorded income is complete.
Designing a numbering scheme
A good scheme is boring: it starts somewhere, goes up by one, and never repeats.
| Scheme | Example | Works well when |
|---|---|---|
| Plain sequence | 1001, 1002, 1003 | One business, one invoicing system |
| Prefix plus sequence | INV-1001, INV-1002 | You also issue credit notes or pro formas with their own prefixes |
| Series per branch or entity | LDN-0412, MAN-0198 | Several sites or trading names invoice independently |
| Year-based | 2026-0001 | Teams that like annual resets; each year’s sequence must still be unbroken |
Points to settle before you issue the first invoice:
- Separate series for separate document types. Credit notes are best numbered in their own sequence (CN-0001), so a credit note never fills a gap in the invoice sequence.
- Pro forma invoices and quotes are not invoices. Give them a different prefix, or no number from the invoice series at all, because they do not create a tax point.
- More than one series is allowed by the “one or more series” wording, for example one per branch, as long as each series is continuous and no number can identify two documents. Keep a note of which series exist and what each is for.
- Let the software allocate numbers. Manual numbering in a word processor is where most duplicates and gaps start.
Gaps: why they happen and what to do
| Cause | What to do |
|---|---|
| An invoice was drafted, numbered and then abandoned | Keep it, mark it void or cancelled, and do not send it |
| An invoice was sent in error | Issue a credit note for the full amount; do not delete the invoice |
| Software skipped numbers after an error or import | Record the missing numbers and the reason in a short note in your VAT file |
| Two people raised invoices in parallel in a spreadsheet | Move invoicing into one system; document the duplicated numbers and how each was corrected |
The principle is that every number must be accounted for. A void invoice in the system, with a reason, is better evidence than a missing number with no explanation. Never reuse a number, even for a voided invoice: two different documents with the same number undermine the whole sequence.
Do not renumber after the event
If you discover a gap months later, do not renumber the invoices that followed. Those invoices have already been sent to customers, entered in their purchase ledgers and reported in your VAT returns. Write the explanation down and leave the numbers as they are.
Changing invoicing software
Moving to new software is the most common point for numbering to break.
- Continue the sequence in the new system: if the last invoice in the old system was 1874, the first in the new one is 1875. Most systems let you set the next number.
- If the new system forces its own format, start a clearly different series (for example a new prefix) and record the last number used in the old system and the switch date.
- Do not run both systems for invoicing at the same time, even for a week.
- Keep the old system’s records, or an export of every invoice, for as long as the record retention rules require.
The wider move is covered in opening balances when changing software.
A quick sequence check
Once a quarter, before you submit the VAT return, export the invoice list sorted by number and look for jumps, duplicates and void invoices without a reason. It takes five minutes and catches the omitted invoice before it becomes an understated return; the VAT return evidence pack is a good place to file the result.