Dr Merkel has broken with political tradition and demanded that the German pontiff make "a very clear statement" to reject the observations of Bishop Richard Williamson, who told a Swedish TV program he did not believe gas chambers existed in World War II or that six million Jews were murdered in Nazi concentration camps. In an unprecedented attack on the leader of the Catholic Church, she rejected the pontiff's statement last week apologising for Bishop Williamson's comments, saying she did not believe that the clarification was sufficient. Newspapers in Italy noted that Dr Merkel's intervention was particularly significant because she could "hardly be described as a left-winger sympathiser". Dr Merkel made her comments at a media conference with the President of Kazakhstan, Nursultan Nazarbayev, who is visiting Berlin. She said: "It seems to me that it is of great significance if the Vatican makes a decision that could diffuse the impression that a denial of the Holocaust is possible. I demand a detailed clarification on this issue from the Pope and from the Vatican."

Her comments reflect a growing revolt within the Catholic Church  and more particularly within the German hierarchy  as religious leaders increasingly express their horror at the Pope's decision to rescind the excommunication of four ultra-conservative bishops to mend a 20-year schism between religious factions. Bishop Williamson used his blog site, Dinoscopus, to apologise to the Pope for the anguish he had caused but failed to revoke his observations. Cardinal Walter Kasper, head of the Pontifical Council for Christian Unity, which oversees relations with Judaism, was reported yesterday to have conceded that the issue had been badly managed. This follows comments by a group of German bishops who took an unprecedented step to state a "loss of faith in the Pope" in the wake of the annulment of the excommunications. Cardinal Kasper said he had followed the row "with great anxiety", and while he did not criticise the Pope directly he blamed "a lack of communication" inside the Vatican for the decision. His comments signal deepening tension inside the Vatican over Pope Benedict's close interaction with the Congregation for the Doctrine of the Faith  which he headed for close to two decades  and a lack of discussion with other senior leaders.

Members of the Fraternity of San Pio X first broke with the church in 1988, when the late French archbishop Marcel Lefebvre illegally consecrated the four bishops without the requisite permission of Pope John Paul II. The Lefebvrists, who rejected the modernising moves of the Second Vatican Council, excommunicated themselves and this was later formalised by a Vatican decree. The Pope's decision to rehabilitate the group is underpinned by the desire to shore up the conservative and traditionalist ranks inside the Catholic Church. The Fraternity, which claims around a million supporters compared to more than a billion for the official Catholic Church, categorically rejects the validity of any other religions and conducts Mass solely in Latin. The Pope's press secretary, Father Federico Lombardi, said the Pope had denounced any denial of the Holocaust several times, including during a visit to the Auschwitz-Birkenau extermination camp in Poland on May 28, 2006.