The customer VAT registration number (VRN) must be valid for the country you have selected. With every new customer, HM Revenue & Customs (HMRC) suggest that you check the validity of their VRN. As a preliminary check you can use the EU Commissions website to confirm that the number is valid. To be certain your customer's details are correct HMRC strongly recommend you contact the VAT Helpline on 0300 200 3700.
Please note: If you have been advised by HMRC that a customer's VRN is invalid then you must not continue to use it to zero rate supplies to that customer.
For further information about countries and territories within the EU follow the link to EU country codes, VAT numbers and VAT in other languages.