More than 50 dissident Catholic groups published an unusually frank open letter to Pope Benedict yesterday saying the Church's ban on contraception had been "catastrophic" and urging him to lift it.

The letter was published as a paid half-page advertisement in Corriere della Sera, Italy's largest newspaper, on the 40th anniversary of the late Pope Paul VI's controversial encyclical Humanae Vitae, which enshrined the ban.

While criticism of the Vatican and its views is fairly common in articles and editorials in Italian newspapers, it is unusual for a group to take out paid advertising against the Pope, particularly in a large-circulation mainstream newspaper.

The letter said the Church's anti-contraception policy "has had a catastrophic impact on the poor and powerless around the world, endangering women's lives and leaving millions at risk of HIV".

It also said the encyclical continued to be "a source of great conflict and division in the Church" and because most Catholics use contraception and feel they are not sinning, the policy has been "an utter failure".

Pope Paul's encyclical, written in 1968 at the height of the sexual revolution, has been defended by his successors Pope John Paul and Pope Benedict. Pope Benedict recently defended it as far-sighted and said it was "all too often misunderstood and misinterpreted."