The warning, in a long interview in La Repubblica, Italy's Left-leaning newspaper, comes as the Bavarian-born cardinal, who is the Pope's doctrinal chief, is being viewed as an important late entry for the papacy.

Coming shortly after MEPs refused to approve the Italian politician Rocco Buttiglione as European justice commissioner because of his strong Catholic views on gays and women, his statements may be seen by some as something of a manifesto.

The cardinal was speaking against a backdrop of rapidly declining priestly vocations in Europe which is worrying Church leaders.

Describing the development of a "secular ideological aggression" across the continent as "cause for concern," the cardinal said: "In Sweden, a Protestant minister who preached about homosexuality on the basis of an excerpt from the scriptures was put in jail for a month.

"Secularism is no longer that element of neutrality, which opens up space for freedom for all. It is beginning to change into an ideology which, through politics, is being imposed.

"It concedes no public space to the Catholic and Christian vision, which as a result runs the risk of turning into a purely private matter, so that deep down it is no longer the same.

"In this sense a struggle exists and so we must defend religious freedom against an ideology which is held up as if it were the only voice of rationality, when instead it is only an expression of a 'certain' rationialism."

In contemporary society, said the cardinal, who is the Prefect of the Congregation for the Doctrine of the Faith, God had been pushed "very much into the margins".

"In politics, it seems to be almost indecent to speak about God, almost as it were an attack on the freedom of someone who doesn't believe," he said.

The cardinal added: "A secularism which is just, is a freedom of religion. The state does not impose a religion, but rather provides free space to those religions with a responsibility to civil society."

The society in which we now lived was one where there was a great deal of transformation.

"Negative birth rates and immigration are changing Europe's ethnic make-up. Above all we've gone from being a Christian culture to one of aggressive secularism which at times is intolerant."

He said that even though "churches were emptying" and people were "no longer able to believe," Christian faith was "not dead".

He said he remained convinced of hope's inner strength, even if the future of the Church lay more in "other continents" than Europe.

Cardinal Ratzinger, once viewed as the likely papal "kingmaker", is now believed to stand a good chance himself precisely because of his advanced years.

Vatican watchers maintain that long-serving popes tend to be succeeded by short-lived "interim" papacies.

Despite his arch-conservative popular image, the cardinal often reveals a frankness and readiness to discuss even the most delicate subjects. .

He admitted that the gulf between the Church and its faithful over sexuality was a matter for "further reflection".

He said that the Pill had "separated sexuality from fertility and so has deeply changed the concept of life itself.

"The sexual act has lost its meaning and purpose. . . to the point that all kinds of sexuality have become the equivalents of each other. The main consequence is the placing of homosexuality and heterosexuality on equal terms."

END