Aids workers and some Catholics have hailed remarks by Pope Benedict XVI that the use of condoms can be "justified in some cases".
In a book to be released on Tuesday, the pope said that it was acceptable for male prostitutes seeking to prevent the spread of HIV to use condoms.
The move relaxes one of the Vatican's most controversial positions on the rejection of the use of condoms to combat the virus.
Ahead of the launch of the book, Light of the World: The Pope, the Church and the Signs of the Times, the Vatican newspaper ran excerpts of it on Saturday.
"It is a marvellous victory for common sense and reason, a major step forward towards recognising that condom use can play a vital role in reducing the future impact of the HIV pandemic, Jon O'Brien, head of the US group Catholics for Choice (CFC), said.
CFC is a pro-choice organisation which supports a woman's moral and legal right to follow her conscience in matters of sexuality and reproductive health.
In the 219-page book, the pope also speaks frankly about the possibility that he could resign for health reasons and defends wartime pontiff Pius XII against accusations from Israel that he turned a blind eye to the Holocaust.
He says scandals of sexual abuse of minors by priests were "an unprecedented shock", even though he had followed the issue for years, and says he can understand why people might resign from the church in protest.
But it is the section on condoms in the book, based on a long interview with Peter Seewald, a German Catholic journalist, that marks a crack in the once tightly shut door of church policy.
He cites the example of the use of condoms by prostitutes as "a first step towards moralisation", even though condoms are "not really the way to deal with the evil of HIV infection".
The original German text and the French and English versions of the book refer to a male prostitute but an excerpt in Italian in the Vatican newspaper uses female prostitute.
The pope drew unprecedented criticism from European governments, international organisations and scientists in March 2009 when he told reporters while flying to Africa that condoms would not resolve the Aids problem there but, on the contrary, increase it.
The statement was condemned by France, Germany and the UN agency charged with fighting Aids as irresponsible and dangerous.