Tony Blair will declare himself a Roman Catholic on leaving Downing Street, according to a priest close to him.

Father Michael Seed, who is known for bringing high-profile politicians and aristocrats into the Catholic fold and who says Mass for the Blairs in Downing Street each week when they are in London, made the prediction to friends at a recent memorial service.

Last night, when contacted by The Times, he did not deny his comments, but said he did not know if Mr Blair would ever be received “formally” into the Roman Catholic Church.

To do so he would have to take part in a ceremony called the rite of Christian initiation for adults, followed by confirmation and taking the sacrament of Holy Communion.

Father Seed said: “He’s been going to Mass every Sunday. He goes on his own when he is abroad, not just when he is with his wife and children.”

Another church source said that many of the early saints and martyrs were not baptised. Such people were held to have had a “baptism of desire”.

He said that Mr Blair was a Catholic by desire and that this did not necessitate a formal conversion. “He is an ecumenical Catholic,” said the source. “He is a liberal Catholic. In terms of his private life, he is a Roman Catholic.”

Although technically an Anglican, Mr Blair only “darkened the door” of Anglican churches on state and other formal occasions, he added.

Downing Street would not comment on the suggestion that Mr Blair would declare himself a Catholic. A spokesman said: “This story is always circulating in one form or another. The PM remains a member of the Church of England.”

Mr Blair has always been reluctant to discuss his religious beliefs. Alastair Camp-bell, the former Downing Street communications chief, famously told one interviewer: “We don’t do God.” The Prime Minister has also indicated in the past that he attended Mass so that his family, all Catholics, could worship together.

To receive Mr Blair into the fold would be a triumph for the Roman Catholic Church, which has in the past two decades in particular regained its confidence, recovering from centuries of persecution that followed the Reformation.

Mr Blair has been criticised for receiving Communion at Catholic Mass. Cardinal Basil Hume, the late Archbishop of Westminster, wrote to him in 1996 demanding that he should cease taking Communion at his wife’s church in Islington, although he added that it was “all right to do so when in Tuscany for the holidays . . . as there was no Anglican church near by”.

Mr Blair made it clear that he did not agree with Cardinal Hume’s opinion, writing in a pointed letter to him: “I wonder what Jesus would have made of it.”

Writing in The Tablet, the Catholic weekly, Father Seed described how the Prime Minister had regarded his time in office as akin to a “vocation”.

He first made contact with Mr Blair when the family moved into No 10, and strengthened their links with The Passage, Britain’s largest homelessness centre, attached to Westminster Cathedral. Mr Blair launched the Government’s policy on homelessness there in 1998. Father Seed says that being prime minister is both a cross and a privilege.

Catholic rites

If Father Seed, above, is correct, Tony Blair would have to receive the sacraments of baptism, confirmation and Holy Communion to be received formally

Adults who have been baptised into another Church are formally taught about Catholic beliefs and then profess Catholic faith

A Catholic friend would “sponsor” them through this

The formal ceremony, the Rite of Christian Initiation for Adults, is immediately followed by confirmation and then Holy Communion

For a non-Christian, this process would take two years. For a baptised Anglican who has regularly attended Mass for years it would take a year or less

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