Tony Blair's conversion to Catholicism has been criticised by commentators who argue his views as PM were at odds with church teachings.
Tony Blair has met Pope BenedictMr Blair was welcomed into the Roman Catholic church by the Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor - leader of the Roman Catholics in England and Wales.
Cardinal Murphy-O'Connor welcomed the politician's personal decision, which culminated in the ceremony at the chapel of the Archbishop's House in Westminster.
He said: "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."
The move comes after years of speculation that Mr Blair, whose wife Cherie and four children are Catholic, would convert from Anglicanism after he resigned from Number 10 in June.
Converting while in office would have caused him problems in connection with issues such as abortion, contraception, homosexuality and faith schools.
The Society for the Protection of Unborn Children (SPUC) reacted with surprise to the news.
John Smeaton, SPUC's national director, said: "During his premiership Tony Blair became one of the world's most significant architects of the culture of death, promoting abortion, experimentation on unborn embryos, including cloned embryos, and euthanasia by neglect.
"SPUC is writing to Tony Blair to ask him whether he has repented of the anti-life positions he has so openly advocated throughout his political career."
Tory MP Ann Widdecombe, who converted to Catholicism in 1993, told Sky News it was possible in her opinion to be a practising Catholic and prime minister.
She said: "I think the crucial thing to remember is at the point you are received (into the Catholic church) you have to say individually and out loud 'I believe everything the church teaches to be revealed truth'.
"And that means if you previously had any problems with church teaching, as Tony Blair obviously did over abortion, as he did again over Sunday trading...you would have to say you changed your mind.
"And I think people will want to know that he did go through that process, because otherwise it will seem as if the church did make an exception for somebody just because of who he is."