The Vatican emphasised today that the use of condoms was acceptable only in 'exceptional' cases, after an apparent U-turn by Pope Benedict XVI on a blanket ban on their use.

Vatican spokesman Federico Lombardi said the 83-year-old pope was speaking about "an exceptional situation" when he said in an interview that condom use was acceptable to the Roman Catholic Church in "certain cases".

"The pope considered an exceptional situation in which the exercise of sexuality is a real danger to the life of another," said Lombardi.

In a book of interviews due out on Tuesday, Benedict says that condom use is acceptable "in certain cases", notably to reduce the risk of HIV infection, apparently softening his once hardline stance.

The pope used the specific example of a male prostitute using a condom to illustrate his apparent shift in position.

"There may be justified individual cases, for example when a male prostitute uses a condom, where this can be ... a first bit of responsibility, to re-develop the understanding that not everything is permitted and that one may not do everything one wishes," Benedict was quoted as saying.

However, the Vatican spokesman said that "in this particular case, the pope does not morally justify the disordered exercise of sexuality, but considers that the use of condoms may be a 'first act of responsibility'."

The United Nations Programme on HIV/AIDS, UNAIDS, welcomed Benedict's remarks as "a significant and positive step forward taken by the Vatican."

Until now, the Vatican had prohibited the use of any form of contraception -- other than abstinence -- even as a guard against sexually transmitted disease.

In a series of interviews published in his native German, the 83-year-old Benedict is asked whether "the Catholic Church is not fundamentally against the use of condoms."

"It of course does not see it as a real and moral solution," the pope replies.

"In certain cases, where the intention is to reduce the risk of infection, it can nevertheless be a first step on the way to another, more humane sexuality," said the head of the world's 1.1 billion Catholics.