The Vatican has ordered an ultra-traditionalist bishop to publicly recant his views denying the Holocaust.
A statement said British Bishop Richard Williamson must "unequivocally" distance himself from his statements to serve in the Roman Catholic Church.
The Vatican also said that the Pope had not been aware of the bishop's views when he lifted excommunications on him and three other bishops last month.
Earlier, a senior cardinal acknowledged the Vatican had mishandled the issue.
The Pope's decision, ending Bishop Williamson's excommunication on an unrelated matter, has caused a bitter row, as the bishop does not believe that Jews were gassed by the Nazis in World War II.
The BBC's David Willey in Rome says it is almost unheard of for a pope to admit publicly that he has made a mistake. But that is in effect the significance of the urgent statement put out by the Vatican, our correspondent adds.
The Vatican should clarify unambiguously that there can be no denial
"Bishop Williamson, in order to be admitted to the Episcopal functions of the Church, must in an absolutely unequivocal and public way distance himself from his positions regarding the Shoah [Holocaust]," it said.
It said Bishop Williamson's positions on the Holocaust were "absolutely unacceptable and firmly rejected by the Holy Father".
On Tuesday, German Chancellor Angela Merkel urged the Pope - who is also German - to make a clearer rejection of Holocaust denials.
"This is not just a matter, in my opinion, for the Christian, Catholic and Jewish communities in Germany but the Pope and the Vatican should clarify unambiguously that there can be no denial," she said.
The Pope had been unaware of the bishop's views, the Vatican said
Cardinal Walter Kasper, who is in charge of relations between the Roman Catholic Church and Jewish leaders, admitted different parts of the Vatican administration had not talked enough to each other, and failed to check where problems could arise.
Bishop Williamson - who was ex-communicated 20 years ago on an unrelated matter - has apologised for stirring controversy, but not repudiated his views.
Last November, the British-born bishop angered Jewish people across the world when he told Swedish TV: "I believe that the historical evidence is strongly against, is hugely against six million Jews having been deliberately gassed in gas chambers as a deliberate policy of Adolf Hitler."
"I believe there were no gas chambers [during World War II]."
Bishop Williamson said that "revisionists" of the Holocaust believed that up to "300,000 Jews perished in Nazi concentration camps but not one of them by gassing in a gas chamber".
He is one of four bishops, who are members of the Society of Pius X, whose excommunication was lifted last month by the Pope.
The Vatican's statement on Wednesday also said that the society must recognise the reformist Vatican II Council of 1962-65 and the popes who followed it.
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