In a break with his traditional teaching, Pope Benedict XVI has said the use of condoms is acceptable "in certain cases", in an extended interview to be published this week.

After holding firm during his papacy to the Vatican's blanket ban on the use of contraceptives, Benedict's surprise comments will shock conservatives in the Catholic church while finding favour with senior Vatican figures who are pushing for a new line on the issue as HIV ravages Africa.

The comments were made in a book-length interview with a German journalist, Peter Seewald. In the case of a male prostitute, says Benedict, using a condom to reduce the risk of HIV infection "can be a first step in the direction of moralisation, a first assumption of responsibility, on the way toward recovering an awareness that not everything is allowed and that one cannot do whatever one wants".

Contraception can be "a first step in a movement toward a different way, a more human way, of living sexuality," the pope says.

Excerpts from the book, Light of the World: The Pope, the Church, and the Signs of the Times, were published yesterday by L'Osservatore Romano, the official Vatican newspaper. The pope's comments follow his controversial assertion in 2009 that the rising tide of HIV in Africa could be made worse, not better, by the distribution of condoms. He was speaking to journalists as he visited Africa, where the majority of HIV fatalities occur.

At the time, Aids campaigners and European governments expressed outrage. Belgium's health minister said the pope's comments "could demolish years of prevention and education and endanger many human lives".

Francis X Rocca, a Vatican expert and correspondent for Religion News Service, said: "This new statement by the pope is very significant, it is going to shake things up. Even if high-ranking church figures and theologians have come out and said this, it remains a controversial subject and no pope has ever said something like this."

Christina Odone, another leading Catholic journalist and commentator in the UK, described the Pope's comments as a "hugely important moment" which Catholics had spent decades waiting for. "It allows Catholics, when we defend our church, to be able to say that this is a not a church that condemns people to Aids and that this is not a church that wilfully ignores the consequences of having unprotected sex," she said.

Peter Stanford, former editor of the Catholic Herald, described the pope's comments as "very significant. It's a very welcome step if they are facing up to the real issues faced by real people."

Insiders said that word of Benedict's comment spread like "wildfire" at the Vatican yesterday, where he was appointing new cardinals. One said: "People were confused but also excited."

In 2006, the Pontifical Council for the Health Care Pastoral, led by Cardinal Javier Lozano Barragán, was asked by Benedict to report on the use of condoms as a way of combating HIV.

"The pope is saying that if you can prevent disease, the use of condoms could be permissible," said John Allen, senior correspondent for the National Catholic Reporter. "But this has been in the mix for a while," he argued. "I think Benedict has been thinking this way since 2006, which is why he asked for the commission to look into it.

"The problem was not Benedict, it was others in the Vatican who argued that if you said using condoms was OK in certain situations, it would send out the message that they were approved. This was a PR problem."