Pope Benedict XVI has said the use of condoms is acceptable in exceptional circumstances, according to a new book.
He said condoms could reduce the risk of HIV infection, such as for a male prostitute, in a series of interviews given to a German journalist.
But he said a more humane attitude to sexuality, and not condom use, was the proper way to combat HIV infection.
The comments were made in a new book, which the Vatican newspaper ran excerpts of in its Saturday edition.
The Church's hardline stance over contraception has led to the Vatican being heavily criticised for its position in the context of the Aids crisis
The book - Light of the World: The Pope, the Church and the Signs of the Times - is based on a series of interview the Pope gave the German Catholic journalist, Peter Seewald, earlier this year.
"The Church's teaching on contraception predates the discovery of Aids," Mr Ivereigh told the BBC news website.
"The prevalence of HIV raised the question of whether condoms could be used to prevent the transmission of the virus.
"If the intention is to prevent transmission of the virus, rather than prevent contraception, moral theologians would say that was of a different moral order."
UNAIDS, the United Nations programme on HIV/Aids, welcomed the Pope's comments.
"This is a significant and positive step forward taken by the Vatican," said UNAIDS Executive Director Michel Sidibe.
"This move recognizes that responsible sexual behaviour and the use of condoms have important roles in HIV prevention."
On a visit to Cameroon last year, the Pope said the use of condoms could endanger public health and increase the problem of HIV/Aids, rather than help to contain the virus. This drew criticism from several EU states.
Campaigners say condoms are one of the few methods proven to stop the spread of HIV.
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