Former British prime minister Tony Blair has converted from Britain's established church, Anglicanism, to Roman Catholicism, the head of Britain's Catholics said on Saturday.

Blair, whose wife and four children are Catholic, was received into the Catholic Church by Cardinal Cormac Murphy-O'Connor on Friday in a move that had been widely expected after he stepped down from power in June.

"I am very glad to welcome Tony Blair into the Catholic Church," Murphy-O'Connor said in a statement.

"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." Blair, now the Middle East peace envoy, had private talks with Pope Benedict at the Vatican in June and his conversion had been predicted.

He has been receiving spiritual preparation for the conversion from Mark O'Toole, Murphy-O'Connor's private secretary.

His spokesman declined to comment on the announcement, saying it was a private matter.

Last month Blair, who was reticent about his faith during his 10 years in power, said religion was a serious matter for him.

"You know if I am honest about it, yes of course it was hugely important," he said in a BBC documentary.

"You know you can't have a religious faith and it be an insignificant aspect because it's, it's profound about you and about you as a human being." But he added that, while politicians could speak about religious faith in the United States, it was difficult to do so in Britain because "frankly people do think you are a nutter (crazy)".

When once pressed in an interview about his beliefs, his then press spokesman Alastair Campbell famously interrupted and said: "We don't do God." Political commentators have also suggested Blair had been unwilling to make the move while he was still in power because some lawyers believed that 19th century laws could actually prevent a Catholic from becoming prime minister.

It was also thought a conversion could have provoked a conflict with his role in appointing Anglican bishops and he might have also felt the need to tread carefully while mediating in the Northern Ireland peace process between the province's Catholic and Protestant communities.

European Union Trade Commissioner and close political confidant Peter Mandelson said Blair was "not an exhibitionist" about religion but it was very important to him deep inside.

"This is a man who takes a Bible with him wherever he goes and last thing at night he will read from the Bible," Mandelson said.