* Remarks mark significant relaxation in Vatican attitude
* Liberal Catholics, AIDS groups, welcome opening
* Condoms "not a moral solution" but could stem infection
* Opening does not affect birth control ban
(Updates, adds UNAIDS head, Vatican spokesman)
By Philip Pullella
VATICAN CITY, Nov 21 (Reuters) - Liberal Catholics, AIDS activists and health officials on Sunday welcomed Pope Benedict's comments that using condoms may sometimes be justified to stop the spread of the disease.
"It is a marvelous 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, said Jon O'Brien, head of the U.S. group Catholics for Choice.
The pope spoke out in a new book to be published on Tuesday and called "Light of the World: The Pope, the Church, and the Sign of the Times". His remarks, while limited in scope and not changing the Roman Catholic ban on contraception, were nonetheless greeted as a breakthrough.
"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."
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 Jewish accusations 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 quit the Church in protest.
But it is the section on condoms in the book -- a long interview with German Catholic journalist Peter Seewald -- that marked 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 cites a female prostitute.
While some Roman Catholic leaders and theologians have spoken about the limited use of condoms to stop the spread of HIV/AIDS as the lesser of two evils, this is the first time the pope has mentioned the possibility.
"We have never heard this so clearly from a pope's mouth," Vatican spokesman Father Federico Lombardi said, adding, however, that it would be wrong to call it revolutionary.
He said the pope had made a "courageous and original" contribution to the debate about responsible sexuality, while stressing that the use of condoms was not the moral high ground.
In the book, Benedict made clear he did not want to weaken the Church's fundamental opposition to artificial birth control, a source of grievance to many ordinary Catholics.
Last year, the pope caused an international uproar when he told journalists accompanying him to Africa that condoms should not be used because they could worsen the spread of AIDS.
He says that the "sheer fixation on the condom implies a banalisation of sexuality" where this is no longer an expression of love "but only a sort of drug that people administer to themselves".
The Church had been saying for decades that condoms are not even part of the solution to fighting AIDS, even though no formal position on this existed in a Vatican document.
The late cardinal John O'Connor of New York famously branded the use of condoms to stop the spread of AIDS as "The Big Lie".
After Benedict first mentions that the use of condoms could be justified in certain limited cases, such as by prostitutes, Seewald asks: "Are you saying, then, that the Catholic Church is actually not opposed in principle to the use of condoms?"
The pope answers: "It of course does not regard it as a real or moral solution, but, in this or that case, there can be nonetheless, in the intention of reducing the risk of infection, a first step in a movement toward a different way, a more human way, of living sexuality."
Act Up Paris, an AIDS support group, said: "After having said that condoms make the AIDS epidemic worse, after getting involved in questions he has no expertise on, the pope seems to finally be taking account of the principle of reality."
The Vatican's opposition to artificial birth control has been highly contested, even by many Catholics, since it was formalised in the late Pope Paul's encyclical Humanae Vitae (On Human Life) in 1968.
Benedict says that "the basic lines of Humanae Vitae are still correct", indicating that his comments about condoms are not intended to apply to birth control, only to AIDS prevention.
Gerard Guerin, secretary general of Christians and AIDS group in France, said the pope had not gone far enough.
"The pope's statements are convoluted. It's limited to cite only the case of male prostitutes. What are couples with one infected partner supposed to do?" he told Le Parisien newspaper.
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