Former Prime Minister Tony Blair has left the Anglican Church to become a Roman Catholic.
Tony Blair visited Pope Benedict XVI at the Vatican in June
His 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.
But ex-Tory minister Ann Widdecombe - herself a Catholic convert - said Mr Blair's voting record as an MP had often "gone against church teaching".
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: "I am very glad to welcome Tony Blair into the Catholic Church.
"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."
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?'"
But Mr Blair's biographer, Anthony Seldon, said the former prime minister's faith had always been a major influence on his politics.
Mr Seldon said: "He's a profoundly religious figure. Religion brought him into politics in the first place, not reading Labour Party history.
"Catholicism has been the religion of his wife - Cherie Blair has been incredibly important to him throughout his political life, encouraging him to go into politics and adopting many of his positions, so I think it was the obvious part of the Christian faith for him to come into."
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.
Catherine Pepinster, editor of Catholic magazine The Tablet, said the news was not quite the same as if Mr Blair had changed Churches while still prime minister.
"I understand that one of the issues he was concerned with, because he was so closely involved in negotiations over peace in Northern Ireland, that perhaps some people there might have been uncomfortable with the prime minister converting to Catholicism at such a time.
"This situation is different. Although he remains a public figure now, and clearly has a role to play in the Middle East, it isn't perhaps quite the same."
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".
The news of his conversion comes as a document showed Mr Blair had raised "concern" over ongoing business negotiations in a letter about an investigation into a Saudi arms deal.
Days later the probe was dropped, and Mr Blair said the decision to stop it was taken because of national security, and was not linked to commercial interests.
Read a selection of your comments on this story:
I don't think it's right that he selects the head of the Anglican church and then converts to Catholicism.
I think Tony Blair has been a 'closet' Catholic for a long time. I do feel, however, that he should have come out of that closet whilst he was in office. Faith comes before politics and personal ambition. It would have been a brave thing to do and he would probably have lost some popularity. I converted when I was 26 and had to put up with a lot of stick from my family and friends.
Jack Nicholas, Leeds, England
I admire him for returning to the faith of his fathers and for unifying his family. He sets a good example for his four children.
Prof. William Parente, Scranton, Pennsylvania USA
A decent Christian would have established his beliefs before his high office and most certainly not after... I never knew him.
Hugh E Torrance, London England
I am a Catholic myself, and I think it is fantastic that Tony Blair has converted to Catholicism. I also think other prominent Catholics in politics etc, should step back before casting negative comments (Ann Widdecombe..) A persons faith should not be condemned, and as Christians, we should welcome everyone to our family, whatever their past.
Claire, Windsor, UK
I would never have voted for him had I knew he was religious. The thought that people in power have gone to war based upon the voices in their heads fills me with horror. To not believe in fairytales is the norm, anything other than that is delusional. We need more normal atheist people to speak up for common sense.
D. Johnstone, Birmingham, U.K.
Quite frankly, who cares? Not talking about his faith previously was probably one of the few good decisions he took as prime minister. Running the country is not about what version of god you believe in, in fact religion should have no part at all in the day-to-day running of the country.
Alex Bailey, Corby, England
Blair had always made it clear that he had very strong Christian beliefs and that these were brought into play when he made decisions. His Christian zeal came through strongly when he made the decision to invade IRAQ. This was very unhelpful bearing in mind, also, Bush's extreme christian beliefs. It is important in politics, nowadays more than ever before, to appear to appear impartial approach and not attract criticism that one favours a particular religious belief beyond wanting equality and freedom to pursue any religion which respects others. Blair, with Bush, has brought upon the modern world religious strife that will last many many years. Moderates in both Christian and Muslim religions will have been influenced to become radical because of his actions. He should do what he can to repair the damage.
Roger Bacon, Cranleigh Surrey
Yes I have a personal experience of converting, as I was received into the Catholic Church by Father Michael Hollings, an inspirational priest who sadly passed away some years ago but whom I have never forgotten, at the Catholic Chaplaincy of the University of Oxford in 1966. Although I have not always been a 100% practising Catholic over the years (the ups and downs of life have taken their toll) I consider myself a Catholic for ever, and while respecting all the world's religions, I do believe that our faith is the only hope of salvation for the world. Tony Blair will be in my prayers. Many years ago, as a journalist's assistant and translator, I actually once met Tony Blair in Italy, about a year before he became Prime Minister. I was struck by his sincerity and idealism, and I am sure that his conversion is in full harmony with his years of dedication to leadership and the people of Britain and the world. My very best wishes to Tony Blair and his family. May God bless us all.
Rachel M. Costa née Barritt, Pisa, Italy; Lewisham, London
Why didn't he convert while he was in office? I welcome him into the Holy Catholic Church, but i would like to know what his views are on same sex unions and abortion ? . If you support these things then you are not welcome into our Catholic Church .
Bobby McBride, Bowmanville Canada
I am amazed to see how an apparently intelligent person like Tony Blair can fall victim to age old ridiculous efforts to package God, one way or the other. May God have mercy on him and millions of others like him!
Tridib K Biswas, New Delhi, India
No doubt this is a typical Blair 'marriage of convenience'; he doesn't do anything unless it is to his own advantage. Why anyone wants to join any religion, let alone the Catholic one, escapes me.
Graeme Coyne, Sandbach, England
It is between him and his spiritual quest, though all the religions preach the same goodness with different paths. May be he wants to be with the family. N.ALI
N. Ali, Mississauga, Ontario Canada
I do not care about his religion. I care more about his charisma and about his having sent troops to Iraq ... unfortunately.
Peter Goldberger, Vienna, Austria
This is fantastic news, truly wonderful. The only sad thing is, and it is a terrible shame, that he felt unable to convert to Catholicism whilst in office. That is a dreadful indictment of our supposedly free and anti-discriminatory culture.
Dominic Cooper, Birmingham, UK
Congratulations to Mr Blair in taking this courageous step to become a Roman Catholic. He must have been thinking about it for a long time before he made the final decision. May the Lord guide him in his work in the Middle-East & bring peace to that country as he did in Northern Ireland. Regards Liz Hanley
Liz Hanley, Ballybofey Co. Donegal. Rep. Of Ireland.
My father converted to the Catholic Church years after being married to Mom and after several of us were born and raised in the Church. He was a good example of the Catholic faith, and always accepted the faith. He raised 6 of us in the Church. He passed away last year and we had a wonderful service in the local Catholic Church here. I'm proud of him and glad he became a Catholic and raised us as Catholics as well. I'm proud of our Church and accept it entirely. It's good to hear such stories these days. Thank you for printing this. Merry Christmas. Thomas Brown (9th generation immigrant of Irish decent)
Thomas Brown, Mount Hope, WV, USA
Many years ago i converted from being a Christian to an Atheist. The fact that Tony Blairs' change in superstitious beliefs makes it to the top story on the BBC news website is somewhat of a mystery. I thought Tony would have the common sense to denounce all this non-sense, obviously not.
Nigel Martin, Buckinghamshire UK
I converted to Catholicism in 2004, after years in the atheist wilderness, and am getting so much more from my life. It was a brand new life when I converted, with God at my side, and I've never been so fulfilled and happy. I, for one, welcome Mr Blair to our family and wish him and his family a very loving and peaceful life.
Rob, Austin, TX
As a fellow Catholic I wish Tony Blair all the best as he will surely need Divine Assistance if anyone ever did to carry out his mission in the Middle East. I welcome him as I would anyone who makes such a heartfelt and personal decision. His Conversion whilst obviously news should be and is a private matter for him and his family.
Craig Earley, Manchester