The Vatican has drafted new guidelines to reaffirm policy on homosexuality and keeping gays and "supporters of gay culture" out of the priesthood.
The document confirms the Pope's tough stance on homosexuality
Men who experience a short-lived phase of homosexuality may join a seminary if they have overcome such tendencies for at least three years, the draft says.
The document, due to be released next week, has been published online by an Italian Catholic news web site, Adista.
The new guidelines were prompted by a 2002 sex abuse scandal among US clergy.
They make no reference to current priests, but only to those about to join a seminary. A church official who asked to remain anonymous confirmed the authenticity of the document to AP news agency.
Critics have already objected that gay seminarians might feel they have no choice but to lie about their sexual orientation.
Others say the new rules might lead to a dramatic drop in the number of vocations, especially in countries like the USA, where more than 25% of Catholic priests are believed to be non-practising homosexuals.
The five-page document, drafted by the Vatican's Congregation for Catholic Education and approved by Pope Benedict on 31 August, describes homosexual acts as "grave sins" that cannot be justified under any circumstances.
"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," it says.
"Such persons in fact find themselves in a situation that presents a grave obstacle to a correct relationship with men and women."
But the paper also stresses the Church's deep respect for homosexuals, who, it says, should by no means be discriminated against.
Homosexuals had already been barred from priesthood in a 1961 document.
The current guidelines are the outcome of a review ordered by Pope John Paul II in the wake of the highly-damaging abuse scandals in the USA, in which several men accused priests of having abused them as teenagers.