The father of Prime Minister Boris Johnson has told a French radio station he is applying for French citizenship.

Boris Johnson's father is applying for a French passport at the same time his son is leading Britain out of the EU. 

Stanley Johnson told broadcaster RTL on Thursday that he was in the process of "reclaiming" his French identity.

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"It is not a question of becoming French. If I understand correctly, I am French. My mother was born in France. Her mother was completely French, as was her grandfather," he told RTL, which said Mr Johnson, 80, is putting together a French citizenship request.

"So for me it is a question of reclaiming what I already have."

Mr Johnson is a former member of the European Parliament who backed remaining in the EU in Britain's 2016 membership referendum. He has since expressed support for his son as the prime minister led the UK out of the bloc.

Once Britain leaves the EU at 11pm on Thursday, Britons will lose the automatic right to live and work in the 27 EU countries, but those with dual nationality will still be able to do so.

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"I will always be European. That is certain," Stanley Johnson said.

"You cannot tell the English, 'You are not European'. Europe is always more than the common market, more than the European Union. But having said that, yes, having a link like that to the European Union is important."