Cardinal [Cormac] Murphy O'Connor, who is the head of Catholics in England and Wales, said: "I am very glad to welcome Tony Blair into the Catholic Church.PHOTO: L'Osservatore Romano/File
"For a long time he has been a regular worshipper at Mass with his family and in recent months he has been following a programme of formation to prepare for his reception into full communion.
"My prayers are with him, his wife and family at this joyful moment in their journey of faith together."
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."...
Mr Blair's ex-spokesman, Alastair Campbell, once famously told reporters "We don't do God". But reacting to news of Mr Blair's conversion, Mr Campbell said: "I can't say it surprises me at all. His faith does matter an awful lot to him.
"It's something I suspect he probably felt he couldn't do when he was prime minister and he's done it now."
Mr Blair last year told ITV1 chat show host Michael Parkinson he had prayed while deciding whether to send troops into Iraq.
"In the end, there is a judgement that, I think if you have faith about these things, you realise that judgement is made by other people... and if you believe in God, it's made by God as well," he said.
And earlier this year, he told the BBC that he had avoided talking about his religious views while in office for fear of being labelled "a nutter".