The Portuguese tax authorities can insist Portuguese companies withhold 25% of the payment, when they paying invoices submitted by British companies who have sold them goods or services.
To avoid this retention, the Portuguese tax authorities need the Mod 21 document in order to confirm that the British company is based in the UK and pays its taxes in the UK.
If you need to provide the Mod 21 to a Portuguese company in order to get paid the full amount, please download the document and send it to HMRC for them to stamp, and also request a Certificate of Residence from them. The Certificate of Residence is valid for 1 year.
Send the form to:
Corporation Tax Services
HM Revenue and Customs