Former prime minister Tony Blair has left the Anglican Church to become a Roman Catholic. His wife and children are already Catholic and there had been speculation he would convert after leaving office.
Cardinal Cormac Murphy O'Conner, who led the service to welcome Mr Blair, said he was "very glad" to do so.
Last year, Mr Blair, who is now a Middle East peace envoy, said he had prayed to God when deciding whether or not to send UK troops into Iraq.