The news that Tony Blair has adopted Catholicism ends years of purely private worship

OUT of office and out of danger of igniting a constitutional crisis, former prime minister Tony Blair, after years of worship as a closet Roman Catholic, has announced his formal conversion to the faith.

During mass at the Archbishop's House in Westminster on Friday, Blair was received into the Church and given full communion. Cardinal Cormac Murphy O'Connor, who led the service, said the new convert had been a regular at mass with his family and in recent months had been following a programme to prepare him for the "reception into full communion". He added: "My prayers are with him, his wife and family at this joyful moment in their journey of faith together."

Born and brought up an Anglican, Blair's journey to Catholicism has been extraordinary. Those close to him believe that even as far back as his student days at Oxford, when he first met his Catholic wife, Cherie, the decision to convert was put off to avoid political prejudices Blair came to believe could affect his career.

Although Iain Duncan Smith and Charles Kennedy led the Tories and LibDems as Catholics, Blair kept his religious leanings quiet. While there is nothing in British law preventing a Catholic from being prime minister, many constitutional experts believe the PM's role in appointing Anglican bishops, combined with the duty of advising the sovereign, who is the head of the Church of England, creates a potential conflict for a Catholic, who, as a point of faith, takes guidance from the pope.

Blair's contradiction is that he practised his faith in private, yet preached political boldness in public. As New Labour's leader, he ripped apart the traditions of the party, and maintained Labour were best at their boldest. Yet Blair, who once said he regarded Jesus "as a moderniser", kept his faith quiet.

Blair's biographer, Anthony Seldon, claims "faith has always had a major influence on his politics". Seldon also hints that Blair and the Catholic Church have been close companions for longer than most people believe: "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."

Since leaving Downing Street, Blair has gone through the formal process of conversion - evangelisation, acceptance, election, the period of purification and enlightenment, then full communion - but the unanswered question remains as to why Blair felt he could not be both prime minister and a Catholic.

Blair was first noticed at mass in Westminster Cathedral, both with his family and alone, in the years between becoming Labour leader in 1994 and winning the 1997 general election, but it is believed he had been attending mass since soon after his marriage in 1980.

He became an MP in 1983, and a frontbench spokesman the following year. If he believed the top job in British politics would some day be his, did he decide open and public Catholicism would have to wait till he left office?

Being a "private" Catholic did, however, have its awkward moments. Before the 1997 election, Blair regularly took communion at his local Catholic church in Islington. Technically, though, he was breaking the rules. Being leader of the opposition did not constitute "a grave and pressing spiritual need" - the exemption that allows non-Catholics to take part in mass. Blair was also doing much the same thing at Westminster Cathedral, and Cardinal Basil Hume even wrote to Blair to ask him to stop attending because he was not a Catholic.

Was Hume telling Blair he should go public with his faith and become Britain's first Catholic prime minister?

Blair, however, found a way to practise his Catholicism in private and regularly attended church when he was at Chequers. Father Michael Seed is said to have been the priest who regularly visited 10 Downing Street - but by the back door - as Blair's spiritual adviser.

In public, and on the advice of Alastair Campbell, his communications chief, Blair "didn't do God". On the eve of the Iraq war, Blair wanted to end a broadcast with "God bless", but Campbell persuaded him to stick to a secular message.

On TV last year, Blair told Michael Parkinson he had prayed before sending British troops to Iraq. More recently, though, Blair said he avoided talking about his religious views while he was in office for fear of being labelled "a nutter".

So after more than two decades of almost secret worship, and of never taking on the one reform that would have ensured him a legacy - becoming the first Catholic prime minister - Blair will shortly return to Jerusalem as Middle East envoy, would-be peacemaker, and Catholic.

The Archbishop of Canterbury, Dr Rowan Williams, wished Blair well in his spiritual journey. He added: "A great Catholic writer of the last century said that the only reason for moving from one Christian family to another was to deepen one's relationship with God. I pray this will be the result of Tony Blair's decision in his personal life."