On reflection, that is cheating, as Blair didn't officially convert until he left office. The issue is that the Church of England is still the established Church in England and the PM has to advise the Queen on the appointment of bishops. Blair had been attending Mass for years though and receiving instruction and had committed to raising his children in the Catholic faith. Apparently he, Hume and, subsequently, Murphy- O'Connor agreed that the whole constitutional thing was too complicated and that he wouldn't officially convert until after he left office. A Jew (religious practice not race, Disraeli was an Anglican convert), Roman Catholic or Anabaptist apparently cannot therefore be Prime Minister of the UK but no such constitutional bar exists in respect of a Muslim, atheist, Unitarian or Presbyterian. Probably an area of the constitution that needs tidying up.Didn't he convert after being PM?