UK rules

Is WhatsApp invoicing legal in the UK?

Yes. UK rules set out what an invoice must contain — not which channel you use to send it. A properly detailed invoice is just as valid over WhatsApp as by email or post.

Why it's fine

Validity is about contents, not channel.

There is no UK rule that requires invoices to be sent by a particular method. What matters is that the invoice includes the right information and that you keep proper records of your sales. Deliver that same invoice over WhatsApp and nothing about its standing changes.

For VAT-registered businesses there are extra details a VAT invoice must show, but again — that is about the content of the invoice, not how it reaches the customer.

The checklist

What a valid UK invoice includes.

DetailNotes
Your business name and addressPlus contact details.
Unique invoice numberSequential, no gaps.
Invoice dateAnd supply date if different.
Customer name and addressWho is being billed.
Description and amountWhat was supplied and the total due.
VAT detailsRequired if VAT-registered — VAT number, rate and amount.
This is general information, not legal or tax advice. WOPA generates invoices with these fields, and keeps a record of each one you send.