The Vatican has welcomed Tony Blair's decision to become a Roman Catholic.
A spokesman said such an "authoritative personality" choosing to join the Catholic Church "could only give rise to joy and respect".
The ex-PM was received into the Church by the Archbishop of Westminster, Cardinal Cormac Murphy O'Connor.
It comes as research by Christian Research suggests Catholic churchgoers now outnumber Anglicans for the first time since the Reformation in the UK.
Ex-Tory minister Ann Widdecombe - herself a Catholic convert - said Mr Blair's voting record as an MP had often "gone against Church teaching".
Mr Blair's wife and children are already Catholic and there had been speculation he would convert after leaving office.
Cardinal Cormac Murphy O'Connor, 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.
And one of Mr Blair's final official trips while prime minister was a visit to the Vatican in June where he met Pope Benedict XVI.
Mr Blair was received into full communion with the Catholic Church during Mass at Archbishop's House, Westminster, on Friday.
Cardinal Murphy O'Connor, who is the head of Catholics in England and Wales, said he was "very glad" to welcome Mr Blair into the church.
"My prayers are with him, his wife and family at this joyful moment in their journey of faith together," he said.
Chief Vatican spokesman Federico Lombardi said the Catholic church in Rome shared Cardinal Murphy-O'Connor's "satisfaction".
"The choice of joining the Catholic church made by such an authoritative personality can only arouse joy and respect," Fr Lombardi added.
BBC correspondent David Willey said it had been no secret in Rome that Mr Blair had been taking instruction from a Catholic priest as a prelude to conversion.
He added that the Pope was informed of Mr Blair's intentions prior to his visit to the Vatican in June 2007, shortly before he left office.
The Archbishop of Canterbury Dr Rowan Williams, leader of the Anglican church, wished the former prime minister well in his spiritual journey.
He said: "Tony Blair has my prayers and good wishes as he takes this step in his Christian pilgrimage."
Downing Street confirmed the former prime minister had converted but said it was a private matter and it would not comment further.
But Miss Widdecombe, who became a Catholic in 1993, told the BBC Mr Blair's move raised some questions.
"If you look at Tony Blair's voting record in the House of Commons, he's gone against Church teaching on more than one occasion. On things, for example, like abortion," she said.
"My question would be, 'has he changed his mind on that?'"
There has never been a Roman Catholic prime minister of Britain, although there is no constitutional barrier to such a move.
However, it had in the past been suggested that Mr Blair would wait until after leaving office, to avoid possible clashes such as over his role in appointing Church of England bishops.
A study by the organisation Christian Research has found Church of England services are no longer Britain's most popular form of worship and have been overtaken by Catholic mass.
The numbers have swelled due to the large number of EU nationals from Eastern Europe who have immigrated to the UK in recent years, it says.
Estimates for church attendances in 2006, based on previous years' figures, reveal 861,800 Catholics attended services every Sunday compared with 852,500 Anglican worshippers.