Quick guide

What you see	Likely reason, from the sources	What the sources say to do
The service rejects a GB number in the EU tool, or an EU number in the GB tool	Each tool covers different prefixes	Use the GB checker for GB numbers and the EU checker for XI and EU numbers (HMRC)
"Not long enough" or "too long"	HMRC's pattern: GB and XI have the prefix plus 12 or 15 digits; EU numbers 3 to 17 characters	Check the number against the holder's paperwork (HMRC pattern)
Wrong country code	HMRC's field accepts only XI or an EU country code	Re-check the prefix
The number is valid but no name appears	The holder has not agreed to disclosure	Ask the business; the holder can update consent (HMRC)
"Invalid"	The sources do not say why. The Commission page says a number can be invalidated at request or when activity ceases	Ask the business, or the registering authority (search)
A new number is not found	The guidance says new numbers may not yet be in the electronic systems	Wait and try again; late applications can delay processing

Spaces and capitals

HMRC's input pattern accepts a number with or without spaces and in either case. The format checker on this site also ignores spaces, dots and hyphens and turns letters into capitals before testing.

A note on length

HMRC's page says GB and XI numbers are 14 characters, for example GB123456123456, and in the same text says the prefix is followed by 12 or 15 digits. These do not agree with each other. The format checker accepts 12 or 15 digits, to avoid rejecting a valid number, and marks the point as open on the sources page.

Similar-looking identifiers

A VAT number, an ad hoc number and a third-country identifier are not EORI numbers. The guidance says ad hoc numbers cannot be used in summary declarations.

Still stuck

Contact the customs authority that issued the number. For a GB number, that is HMRC, and for an EU number the registering authority of the EU country.