Mr Le Pen has been accused of trying to whitewash Nazi crimes
Leading Euro MPs are pushing for a rule change to stop the veteran French far-right leader, Jean-Marie Le Pen, from presiding over the European Parliament.
The head of the conservative EPP, the largest bloc, has backed the initiative of the Socialists and Greens to block Mr Le Pen, AFP news agency reports.
As the oldest MEP, Mr Le Pen, 80, could chair the new parliament's inaugural session on 14 July under the rules.
Mr Le Pen has again called the Nazi gas chambers "a detail of history".
The Nazis murdered at least six million Jews in death camps in World War II. Hundreds of thousands of others were also killed there, including Roma gypsies and disabled people.
"I merely said that the gas chambers were a detail of the history of the world war, which is an obvious fact," Mr Le Pen, leader of the National Front (FN), told the European Parliament on Wednesday.
His remark drew boos and whistles from MEPs. He was fined 1.2m francs ($290,000) for the same claim in 1987.
Earlier, the head of the Socialist group, German MEP Martin Schulz, said he was "concerned that a Holocaust denier could preside over the inaugural session" on 14 July.
They'll probably also adopt a resolution to stop the Earth rotating
The parliament reconvenes after the European elections in June, in which Mr Le Pen, a staunch nationalist, is expected to run again.
The rules say the oldest MEP - or "doyen" - chairs the session until a new parliament president is elected. But the Socialists and Greens proposed a rule change to block Mr Le Pen.
The Greens co-president, Daniel Cohn-Bendit, said "we would like to see the youngest deputy open the session, not because of Le Pen, but because it's a sign of the future".
Mr Le Pen described their manoeuvre as "idiotic". "It's just like them to do this. They'll probably also adopt a resolution to stop the Earth rotating," he said.
A rule change would require the agreement of all the political groups' leaders and a vote in the assembly.
But there was some dissent. Liberal ALDE leader Graham Watson - speaking before Mr Le Pen repeated his gas chambers statement - said: "There is no reason to treat Mr Le Pen differently from others, even if we hate his politics".