If your customer's VAT registration number has been rejected please check the EU country codes and customer VAT number format to ensure it is in the correct format for the relevant Member State.
You can check the validity of VAT numbers for new customers or verify their name and address is correct. To do this follow the link to VAT number validation.
If your customer's registration number is invalid then you must not continue to use it.
Please note: If the UK leaves the EU with no deal, the rules on EC Sales lists will change. Further information can be found in VAT for businesses in a no-deal Brexit.