The Vatican has angered gay rights groups by reiterating its stand against sexually-active gays joining the priesthood, in a doctrinal paper approved by the Pope.

But the Instruction, due to be released next week but leaked by a Catholic news agency, allowed those whose homosexuality is "transitory" to seek ordination if they have been celibate for at least three years.

One US expert said that the Church risked losing "many, many gay men who are fabulous priests".

The five-page document from the Vatican's Congregation for Catholic Education was approved in August by Pope Benedict XVI, who in his previous role as head of the Congregation for the Doctrine of the Faith described homosexuality as a "troubling moral and social phenomenon".

Vatican prohibitions on sexually-active gays becoming priests are not new, and a 1961 document says homosexuals should be barred from the priesthood. But the issue came to the fore in 2002, at the height of the clergy sex abuse scandal in the United States.

A study by the John Jay College of Criminal Justice found most abuse victims since 1950 were adolescent boys. Experts on sex abuse said that homosexuals are no more likely than heterosexuals to molest young people, but that did not stifle questions about gay seminarians.

In addition, some Catholic researchers said "gay subcultures" in seminaries were alienating heterosexuals, prompting them to drop out.

The Instruction makes a distinction between "deep-seated" homosexual tendencies and homosexuality that it calls "the expression of a transitory problem".

"Nevertheless, such tendencies must be clearly overcome at least three years before ordination before ordination to the deaconate," it says.

"In order to admit a candidate to ordination to the deaconate, the Church must verify, among other things, that the candidate has reached affective maturity," the document adds.

"If a candidate practises homosexuality, or presents deep-seated homosexual tendencies, his spiritual director as well as his confessor have the duty to dissuade him in conscience from proceeding towards ordination.

"Those people find themselves, in fact, in a situation that presents a grave obstacle to a correct relationship with men and women. One cannot ignore the negative consequences that can stem from the ordination of people with deeply rooted homosexual tendencies."

Gay campaigners predictably attacked the document and said that the Church was once again sidestepping the real issue it has to deal with: paedophilia.

"This looks like a diversionary tactic to deflect public attention away from the Vatican’s real problem, which is child sex abuse by clergy," Peter Tatchell, of the pressure group OutRage!, told Reuters. "The Pope should be tackling paedophiles within the Church, not witch-hunting gay people."

Harry Knox of the American gay rights group Human Rights Campaign accused the Vatican of "using gay people as scapegoats". "It sets a very dangerous precedent," he said. "The church is not acting like Jesus Christ would. Jesus would never exclude."

Although the Instruction will be welcomed by conservatives within the Church's hierarchy and membership, critics have said that it might simply result in seminarians lying about their orientation and further reduce the number of men seeking ordination.

Estimates of the number of gays in US seminaries and the priesthood range from 25 to 50 per cent, according to recent review of the research.

The Vatican document says the church deeply respects homosexuals. But it also says it "cannot admit to the seminary and the sacred orders those who practice homosexuality, present deeply rooted homosexual tendencies or support so-called gay culture".

Thomas Plante, a psychologist who has conducted evaluations of prospective seminarians for US dioceses and religious orders, said that the document would have an "enormous" ripple effect on US seminaries and the priesthood if it is followed.

"Sexual orientation in almost all the evaluations I’ve done over 15 years hasn’t really mattered," he said. "Now what’s coming out of the Vatican is that it matters in a big way. That’s a real challenge because we think that there are many, many, many gay men who are fabulous priests."

He questioned how seminary directors would apply the new regulations, and suggested that many may resort to a "don’t ask, don’t tell" policy. The candidates also may try to hide their sexual orientation, said Mr Plante, who is chairman of the psychology department at Santa Clara University in California.

"That puts us in a bind because, in order to do a real evaluation, you need to have some understanding of their sexual history or behavior, regardless of their orientation," he said.

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