Staff at 10 Downing Street insisted he did not plan to convert, although they did admit that he had been to Mass at the cathedral without his wife and their three children.
The Prime Minister, an Anglican, regularly attends Roman Catholic Mass with his family, but it was not previously known that he had chosen to worship in a Catholic church alone.
A Downing Street spokesman said: "Though the Prime Minister regularly worships at a Catholic church with his wife and family, he is not converting to Catholicism.
"The only occasion on which he has attended Westminster Cathedral alone arose because he had been at a speaking engagement and his family had attended church earlier."
However, the Press Association, which broke the story, said it had spoken to sources who said they had seen Mr Blair alone in the cathedral, except for bodyguards, "on several occasions" since Christmas.
The news agency said he had been seen at the 6pm Saturday Mass, which is often attended by around 1,000 people, and the Sunday 5.30pm Mass, which attracts up to 700 worshippers.
Constitutional experts were agreed that there was no legal obstacle to having a Catholic prime minister - even though Britain has never had one - and that Mr Blair's role in choosing bishops was in his capacity as prime minister and not in a personal capacity.
Obstacles, however, might be presented by other Catholic worshippers opposed to his stance on abortion.
Phyllis Bowman, a founder of the Society for the Protection of Unborn Children and herself a Catholic, said: "You don't join the rugby team if you want to play football.
"Mr Blair has voted in favour of abortion 13 times since he was voted to Parliament in 1984.
"His conscience is his own business but I think his attendance at Mass is hypocritical.
"If he wants to go to Catholic Mass he should embrace its teachings and become a Catholic - and that is incompatible with his political beliefs."