A week ahead of the decisive second round vote on May 6, investigative website Mediapart said it had uncovered a document from Libya's former secret services showing that Gaddafi's government had decided to finance Sarkozy's run at the presidency when he was interior minister.
Sarkozy, whose government had played a key role in the ouster of Gaddafi last year, has repeatedly denied receiving any money from the former Libyan leader, who was captured and killed by fighters from the Libyan National Liberation Army last year.
Sarkozy said that the Socialists were using the report as an attempt to distract attention from the reappearance of Strauss-Kahn, who had been the runaway favorite for the party's presidential ticket before he was arrested last May on charges of sexually abusing a New York hotel maid.
In an interview published by London's Guardian newspaper on Friday, Strauss-Kahn said that operatives linked to Sarkozy had torpedoed his presidential bid by ensuring his sexual encounter with the maid was made public.
"You see that this is an attempt to create a distraction after the return to public life of Strauss-Kahn," Sarkozy told Le Parisien-Dimanche newspaper. "They don't want anyone to remember they wanted to make him the next president of the French republic."
The Socialists' candidate François Hollande leads Sarkozy in opinion polls by around 10 percentage points ahead of Sunday's second round vote.
Mediapart, staffed by a number of veteran French newspaper and news agency journalists, said it had a 2006 document signed by Gaddafi's former intelligence chief Moussa Koussa which stated his government would pay 50 million euros for Sarkozy's campaign.
Later on Sunday, in an interview on Canal+ television, Sarkozy said the document was a fake.
"It's a disgrace. It's a fabrication," he said. "Mediapart is a habitual liar ... It is an office in the service of the left."
"Who led the coalition to topple Gaddafi? It was France! I was perhaps the leader. Do you think that if Gaddafi had anything on me I would have tried to oust him?"
Prime Minister François Fillon said the document was "false, or at least impossible to verify, which comes from a dictatorship which France helped to topple".
"It is a calumny, an absurdity," he told Europe 1 radio, adding that it was ridiculous to talk of 50 million euros to finance a campaign that cost 20 million and for which accounts were publicly available.
The Mediapart website, which gained prominence in 2010 when it broke news of a major political funding scandal around Sarkozy's UMP party and L'Oreal heiress Liliane Bettencourt, called for an official investigation.
Having earlier denied it, Sarkozy acknowledged on Wednesday that his government had considered cooperation with Libya in civil nuclear energy.
He raised eyebrows when he invited Gaddafi to Paris in 2007 - letting him pitch his Bedouin-style tent by the Elysée Palace - following the freeing of five Bulgarian nurses detained in a Libyan jail, largely at the French president's personal intervention.