In an embarrassment to the French President, and with just two days to go before the seventh anniversary of the deadly terrorist strikes, Jean-Marie Bigard - a member of Mr Sarkozy's inner circle of showbiz friends - said the official version was a "lie".
"We are now absolutely certain that these two planes - the one that supposedly fell into a forest (flight 93 in Pennsylvania) and onto the Pentagon don't exist... those planes are still flying.
"It's an enormous lie," he told Europe 1 radio.
"It's an American missile that hit the Pentagon, they provoked it themselves, they killed fellow Americans.
"Usually one learns of these things 30 years afterwards. We know it now," he went on.
Mr Bigard, who has staged shows to a full house in Paris' football stadium, le Stade de France, added: "We are beginning to seriously consider that neither Ben Laden nor al Qaeda were responsible for September 11."
He cited the controversial 2002 bestseller L'Effroyable Imposture (The Big Lie) by Frenchman Thierry Meyssan, which claimed the Pentagon crash was a "fake", as well as the hit internet conspiracy film Loose Change.
His comments were condemned as "grotesque" by former French foreign minister Hubert Védrine. "There will always be people who believe that things are being hidden from them, that everything is being hidden from them," he said.
Europe 1 has issued an apology but Mr Bigard said he stood by his words. The crude and scatological comic, whose humour travels poorly, was one of several celebrity friends Mr Sarkozy took with him to meet the Pope late last year, as he is a devout Catholic.
He is the second French star to accuse America of fabricating the 9/11 attacks.
In March, the Oscar-winning French actress Marion Cotillard angered Americans with a similar conspiracy theory on a chat show.
The 32-year-old, who won a BAFTA award for her portrayal of Edith Piaf in La Vie en Rose, had said that the US regularly lied about major events, up to and including the terrorist attacks on New York.
"I think we're lied to about a number of things," she said, even claiming the Americans had destroyed the Twin Towers themselves because they were an outdated "money sucker".
Expanding her conspiracy theory, Miss Cotillard said it would be "a lot more expensive" to rebuild the towers "which is why they were destroyed," with the loss of thousands of American lives.
She later was forced into an embarrassing climb-down, with her lawyer saying she "never intended to contest nor question the attacks of September 11, 2001," and that her remarks were taken out of context.
"It is almost impossible to fight against this phenomenon," conspiracy theory specialist Pascal Froissart told Le Parisien. "Because the fact of denying it spreads the rumour even further."