Mr Blair's long-expected switch to the faith of his wife and four children is expected to come at a Mass in the private chapel of Cardinal Cormac Murphy- O'Connor, the leader of Roman Catholics in England and Wales.
The Catholic newspaper The Tablet says the service of conversion will take place this month.
Mr Blair has regularly attended Catholic services in recent years, both with his family and alone. He has also visited Pope Benedict XVI and his predecessor John Paul II.
The Tablet said Mr Blair has received instruction from two priests, RAF chaplain John Walsh and Father Mark O'Toole, the cardinal's secretary. A candidate for reception into the Catholic Church is usually received by their parish priest.
The cardinal's involvement, as Archbishop of Westminster, would suggest Mr Blair's process of conversion began while he was in Downing Street.
Mr Blair's spokesman did not deny the report, saying: "This is the same old speculation", but a friend said it was "not without substance". News of the imminent move drew calls from leading Catholics for Mr Blair to renounce his backing in Parliament for abortion up to 24 weeks. (Daily Mail)