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HMRC internal manual

National Insurance Manual

Special Cases: International going to or coming from abroad: EEA Regulations 883/04 (After 1 May 2010): Third country nationals

Regulation 883/2004 of the European Parliament and of the Council of 29 April 2004 on the coordination of social security systems

The United Kingdom does not apply Regulation 883/2004 to third country nationals although most other Member States do.

A third country national, is somebody who is not a national of any of the Member States but who is resident in a Member State

By resident we mean legally resident in a Member State.

If you have a third country national moving within the Member States you should apply the rules in Regulation 1408/71 see NIM33300 

If you see a Portable Document A1 issued to a third country national by the competent authority of another Member State then this should be accepted