Lieutenant Philip Mountbatten married Princess Elizabeth, having renounced his titles as Prince of Greece and of Denmark, having converted to the Anglican church from the Greek Orthodox, and become a naturalised citizen of Britain and taken the surname of Mountbatten, that of his maternal grandparents.

When he married he was granted 'the style' HRH, and was given the title of Duke of Edinburgh, but there was a gap in his being a Royal Highness which was just a little longer than the period of his engagement.

At least that is what Wikipaedia says, and I am fairly sure that is what happened.

Anne the Pleater :ootd: