Sequential numbering: the rule that stops an invoice going missing
It is a one-line obligation in the regulations and the first thing an inspection looks at. A gap in the numbering is a direct question.
The obligation to number invoices consecutively within each series, with no gaps and no repeats.
Article 6.1.a) of the invoicing regulations puts it without decoration: the numbering of invoices within each series shall be sequential. Nothing more, and that is enough for the rest of the accounts to hold up.
Why it matters so much
Consecutive numbering is what makes it possible to check that no invoice is missing. If series A-2026 contains 0041, 0042 and 0044, the question is obvious: where is 0043. And there are only two good answers: it exists and is recorded, or it was cancelled and there is a corrective invoice documenting it.
Cancelling is not deleting. An invoice issued by mistake does not disappear: it is corrected. The number used stays used.
What changes with VeriFactu
The system in RD 1007/2023 goes beyond the number: each invoicing record is chained to the previous one by a hash, so that altering or deleting one breaks the chain and shows. The obligation arrives on 1 January 2027 for Corporation Tax payers and on 1 July 2027 for everyone else.
An example of how it breaks
A garage numbers its invoices by the date they were paid instead of the date they were issued. In March it is paid for the one dated the 20th before the one dated the 12th, and numbers them the other way round. The result: two invoices whose numbering contradicts their own dates, in the same series. There is no gap, but the series stops being sequential in time, which is what gets checked.
The mistake that comes up most
Reusing the number of a cancelled invoice “because it was never sent”. That number is already in the record book and, with VeriFactu, in the chained record. Reusing it creates two different invoices with the same identifier, which is exactly the problem the rules are there to prevent.
Where this carries on in Cairos: How VeriFactu chains the records.
Terms that go with this one
Almost no tax concept makes sense on its own. These three are the ones that most often turn up beside it.
Invoice series
A group of invoices with its own sequential numbering, usually identified by a prefix.
InvoicingCorrective invoice
The invoice that corrects one already issued, in its own series and referring expressly to the invoice being corrected.
AccountingRecord book
The compulsory record, in date order, of the invoices issued and received, which underpins the self-assessments and the annual summaries.
This, handled without thinking about it
Cairos keeps the invoices, the record books and Hacienda's forms from the same data, so the theory on this page turns into boxes that are already filled in.
No card and no minimum term.