He has been attending Catholic mass, often with his family but also occasionally alone, since long before he became prime minister. His wife, Cherie, is a lifelong and practising Catholic, and in accordance with church rules their children have been brought up as Catholics and were sent to church schools.

More than 10 years ago Mr Blair was slipping into Westminster cathedral and occasionally taking communion, until the late Cardinal Basil Hume told him to stop because it was causing comment as he was not a Catholic - an injunction that bemused him at the time.

Since then he has regularly attended services conducted by Canon Timothy Russ, parish priest of the Immaculate Heart of Mary at Great Missenden, the nearest Catholic church to Chequers.

He is also known to have had discussions with priests such as Father Timothy Radcliffe, former head of the worldwide Dominican order, now at Oxford, and with Father Michael Seed, who has shephered a number of high-profile figures, including Ann Widdecome and, allegedly, Alan Clark, towards conversion. Fr Seed, an engaging if indiscreet figure, has claimed to have paid regular backdoor visits to Downing Street to talk religion, if not necessarily to advise the prime minister.

So why has it taken so long? Almost certainly because of Mr Blair's sensitivity about the place of Catholicism in British public - and particularly its constitutional - life. The only positions specifically barred to Catholics are marriage to the sovereign or heir to the throne, or becoming sovereign themselves, a legacy of the Act of Settlement that followed the Glorious Revolution of 1688 and the deposition of the last Catholic monarch, James II; there has never been a Catholic prime minister.

In the last 40 years Catholics have entered many senior positions in British public life, generally without comment except among the wilder fringes of Protestant Calvinism: in the civil service, the Foreign Office and industry, as MPs and ministers in Conservative and Labour cabinets. The current director general of the BBC, Mark Thompson, is a Catholic and, briefly, four years ago, with Charles Kennedy, leader of the Liberal Democrats, and Iain Duncan Smith, leader of the Tories, so were the alternative prime ministers.

But the motives of Catholic politicians have traditionally been regarded with suspicion by non-Catholics, both here and in the US, based on the allegation that they take their orders from the Vatican rather than the electorate. Catholic political leaders have always denied it - but the recent antics of some bishops in the US during the 2004 presidential campaign when they threatened to deny John Kerry communion because of his support for abortion rights and, recently, Cardinal Keith O'Brien's warning that he would do the same in Scotland, have tended to confirm old suspicions.

A number of potentially divisive moral issues would have been much more difficult if Mr Blair had been known to be a Catholic, even though his personal beliefs have not necessarily intruded into the government's decisions.

Ministers have enacted civil partnerships for gay couples and this year faced down demands, particularly from the Catholic church, for exemption from equality provisions enabling gay couples to adopt children, even though the prime minister favoured compromise.

Equally, the government has not attempted to limit abortion rights - an issue regarded as long settled in Britain except by some mainly Catholic groups - or pushed for reduced time limits, even though the church regards abortion as a sin. And it has permitted stem cell research without conceding to Catholic opposition.

Mr Blair, like President George Bush, ignored the condemnations and warnings of the Pope and all other church leaders over the war in Iraq.

He has been keen to expand the number of faith schools and church-supported academies, in the face of strong opposition from secular groups, but here again seemingly not for reasons of religious indoctrination but because of their parental popularity.

The criticism of Ruth Kelly when she was education secretary because of her membership of the lay sect Opus Dei - at a time when the novel The Da Vinci Code had made the group more widely known - also showed that the old prejudice could still be deployed. Mr Blair probably thought he could do without the extra hassle.

He has kept his personal religious views largely out of his political life. Ostentatious religiosity does not go down well in Britain. He dropped his wish to end a prime ministerial broadcast on the eve of the Iraq invasion with the words: "God bless" on the advice of Alastair Campbell, who famously told him "We don't do God".

Explainer: Becoming a Catholic

The path to purification

Converting to Catholicism is not a straightforward or easy process, as Tony Blair will have realised. It takes time - though how long depends on the candidate's readiness and aptitude - and is based on the church's assessment of their sincerity and commitment. The process is described in a 44-page document called the Rite of Christian Initiation.

When there was a rush of conversions from Anglicanism in the early 1990s, after the Church of England's decision to ordain women priests, there was considerable murmuring among lifelong Catholics that the conversion of defectors such as John Gummer and Ann Widdecombe had been too easily sanctioned by Cardinal Basil Hume, the leader of the Catholic church in England and Wales.

That is unlikely to be the case with Mr Blair since his conversion is clearly the result of a long period of consideration and is not due to a particular grievance.

Adults wishing to convert undergo a period of doctrinal and spiritual preparation with a priestly adviser to become catechumens, preparing for admission to the church. They are no longer required to make an abjuration of previous heresy but they do make a profession of faith and belief that they "consciously and freely seek the living God and enter the way of faith and conversion as the Holy Spirit opens their hearts."

The rite says candidates are to receive help and attention, so that "with a purified and clearer intention they may cooperate with God's grace."

The process takes several stages of indeterminate duration: after the period of evangelisation there follows acceptance into the order of catechumens, then election, when the church ratifies candidates' readiness. A "period of purification and enlightenment" follows, usually on the eve of Easter, followed by the sacraments of initiation and then catechesis as the candidates are allowed to participate fully in the sacraments, such as communion.

Although conversions usually take place during the Easter period and in public ceremonies, this need not necessarily be the case if there are special circumstances - which the church could probably find for a former prime minister.