As Pope Benedict XVI began his first visit as pontiff to France on Friday, it emerged in Rome that he has instructed Vatican officials to adopt stricter criteria for the approval of Marian apparitions.

The Pope was greeted at Paris airport at the start of his four-day visit by President Sarkozy and his wife, the Italian-born supermodel-turned-singer Carla Bruni. He then headed to the Elysee Palace for talks with Sarkozy, who last year affirmed France's "Christian values" and called for "a place for religion in public life".

France is overwhelmingly Roman Catholic - at least nominally - but also has large Muslim and Jewish communities and adheres to the separation of church and state. Sarkozy is himself a lapsed Catholic who has been divorced twice.

However, visiting the Vatican at end of last year, Sarkozy launched the concept of "positive secularism" saying "religions should be considered not a danger but an asset." A survey published last year showed 51 percent of the French consider themselves Catholic, down from 80 percent in the early 1990s. Le Monde said the Pope would be "talking to a weakened church on secular ground".

Benedict is to deliver a keynote address on the role of religion in society to the French Academy, of which he has been a member since 1992. Tomorrow, after an open-air Mass in Paris, he flies to Lourdes for celebrations of the 150th anniversary of the apparitions of the Virgin Mary to St Bernadette Soubirous, a peasant girl who is said to have seen the Madonna 18 times in 1858.

The visions of Bernadette are not in doubt, and have long been approved by Rome. However the Pope has asked Monsignor Luis Francisco Ladaria Ferrer, a Spanish Jesuit and secretary of the Congregation for the Doctrine of the Faith - which Benedict himself headed as Cardinal Joseph Ratzinger - to draw up new guidelines for bishops around the world on the recognition of reported apparitions by the Virgin Mary.

Vatican official said the Pope wanted to avoid "excesses and abuses". He believes bishops should resist being swayed by the emotional reaction of believers and be guided instead by strictly applied "scientific, psychological and theological criteria".

Ignazio Ingrao, Vatican correspondent of the weekly news magazine Panorama, said there had been particular controversy over the readiness of Monsignor Girolamo Grillo, the bishop of Civitavecchia, the port north of Rome, to approve reports that a statue of the Madonna owned by a local family had wept tears of blood. The bishop even claimed to have seen the tears himself while holding the statue in his arms.

He was later replaced by Bishop Carlo Chenis, a protege of Cardinal Tarcisio Bertone, the Vatican Secretary of State and Pope Benedict's right-hand man. The Vatican is also sceptical about reported Marian apparitions since 1981 at Medjugorge in Bosnia-Herzegovina, despite the fact that the site is visited by two million pilgrims a year.

Vittorio Messori, an Italian Catholic writer who is close to the Pope, said the then Cardinal Ratzinger had told him in 1985 that "patrience and caution" were the key to validating Marian visions. "No apparition is indispensable to the faith" the future Pope told Messori. "The Revelation ended with Jesus Christ".

He said Benedict's approach to Mariology was a mixture of "the mystical and the intellectual". The Pope had chosen to go to Lourdes rather than Fatima in Portugal, perhaps because of the lingering controversy over the apocalyptic "secrets" said to have been vouchsafed by the Virgin Mary to three shepherd children in 1917.

The unpublished "Third Secret" was revealed by Pope John Paul II in 2000 to be in part a prediction of the attempt on his own life in 1981. But there are persistent reports that the Vatican suppressed another part of the "Third Secret" forecasting the decline and even extinction of the Christian faith in Europe.

Guidelines for the approval of apparitions and revelations were last issued in 1978. They lay down that a diocesan bishop can "either on his own initiative or at the request of the faithful" choose to investigate an alleged apparition. He then submits a report to the Vatican for approval.

Before leaving Rome, Benedict, who speaks fluent French, said he would pray at the feet of Our Lady of Lourdes for the Church, the "sick and abandoned", and peace in the world.

"I go as a messenger of peace and fraternity," he said in a message to the French people. "Your country is not unknown to me. On several occasions I have had the joy to visit it and to appreciate its generous tradition of hospitality and tolerance, as well as the solidity of its Christian faith and its lofty human and spiritual culture."

He added: "May Mary be for all of you, and in particular for young people, a Mother always attentive to the needs of her children, a light of hope that illuminates and guides your ways."