Pope Benedict XVI on Friday reaffirmed the Catholic Church's condemnation of artificial birth control, a position that has driven millions of people away from the faith.

Contraception "means negating the intimate truth of conjugal love, with which the divine gift (of life) is communicated," the leader of the world's 1.1 billion Roman Catholics wrote on the 40th anniversary of a papal encyclical on the controversial topic.

The message came two months after an appeal for a retraction by some 60 Catholic groups who said the Church's stance had been "catastrophic" for the world's poorest and weakest.

The open letter in July by dissident Catholic bodies from countries including Britain, Brazil, Canada, France and the United States said the Church's opposition to birth control endangered women's lives and exposed millions of people to the risk of contracting AIDS.

It said the impact of the 1968 encyclical had been "disastrous in the southern hemisphere, where the Catholic leadership exercises considerable influence on the politics of family planning."

An encyclical is a letter usually treating some aspect of Catholic doctrine and issued occasionally by the pope.

The landmark document, whose title in English is "On the Regulation of Birth," was published at a time when the development of the Pill was giving new sexual freedom to women across the world.

Millions of Catholics distanced themselves from Rome as a result, while the clergy were divided on how to deal with such a document, covered as it was by the doctrine of papal infallibility.

The 81-year-old pope's message on Friday to a seminar on the encyclical also reaffirmed that the rhythm method is an acceptable form of contraception for couples in "dire circumstances" who need to space their children.

The pontiff recognised that "the world and many of the faithful have a lot of difficulty understanding the message of the Church," which he said aimed to protect "the beauty of conjugal love."

"Technological answers often seem the easiest, even to great human questions," he wrote.

“But technology cannot substitute for a mature approach to freedom when love is involved,” he added.

It was for the same reason, he said, that the Catholic Church advocates "natural solutions to infertility."

The Vatican opposes assisted procreation through donated sperm or eggs and the freezing of embryos.

In July, the Vatican said the idea that its stance on contraception contributed to the spread of AIDS was "obviously unfounded."

Vatican spokesman Federico Lombardi said the July appeal was not about expressing a "theological or moral position but paid propaganda in favour of the use of contraceptives."

"The spread of AIDS is totally independent of the religious denomination of populations and of the influence of religious hierarchies," Lombardi said, adding that policies aimed at preventing the spread of HIV through condom use had largely failed.

French Cardinal Roger Etchegaray wrote in his memoirs that opposition to the encyclical within the Church was a "silent schism" that had weakened papal authority.

Large majorities of American Catholics told opinion pollsters in April 2005, when Benedict was elected pope, that they wanted the next pontiff to allow them to use birth control.