French President Nicolas Sarkozy has filed a lawsuit against a news website over an article alleging that Muammar Gaddafi's government sought to fund his 2007 election campaign.
Left-wing investigative website Mediapart published a letter, apparently signed by the former head of Libyan intelligence, offering up to €50m funding.
Mr Sarkozy faces Socialist Francois Hollande in the second round of the presidential election next weekend.
The current president is clawing back points from Mr Hollande, whose own presidential bid has been hit by the intrusion of disgraced IMF chief Dominique Strauss-Kahn into the campaign.
Both candidates have been appealing to the 18% of voters who chose anti-immigrant candidate Marine Le Pen in the 22 April first round, with Mr Sarkozy riding on the back of rhetoric inspired by her National Front party.
Yesterday, Mr Sarkozy dismissed the document published by Mediapart as a "crude forgery".
"We will file a suit against Mediapart... This document is a crude forgery, the two people supposed to have sent and received this document have dismissed it," Mr Sarkozy told France 2 television.
Mr Sarkozy and his supporters believe that he is relentlessly targeted by "biased" left-wing media.
"There's a section of the press, of the media, and notably the site in question whose name I refuse to mention, that is prepared to fake documents. Shame on those who have exploited them," he said.
Paris prosecutors later in the day opened a preliminary inquiry into Mediapart following Sarkozy's complaint, judicial sources said.
Claims that Gaddafi financed Sarkozy's 2007 campaign are not new, but Mediapart's document bearing the signature of Libya's former foreign intelligence chief Moussa Koussa is.
The letter was addressed to Bashir Saleh, Gaddafi's former chief of staff and head of Libya's $40 billion sovereign wealth fund, who is currently resident in France.
But Mr Saleh's lawyer said he had "grave reservations" about the document while Mr Koussa, who lives in Qatar, said: "All these allegations are false."
Mr Hollande said that it was up to judges to decide on the allegations.
"If it's a fake then the site will be found guilty, if it's not a fake then he (Sarkozy) will have some explaining to do," Mr Hollande said.
Socialist Party spokesman Benoit Hamon took a stronger line and said: "We call on the justice system to take up these disclosures by the Mediapart site."
"Obviously Nicolas Sarkozy has a complicated history with Mr Gaddafi," he said, speaking at a campaign event for French voters in Algeria, allowing however that "the facts are yet to be verified".
Mr Hollande's team meanwhile sought to play down the re-emergence of one-time presidential hopeful Strauss-Kahn after he attended a Socialist politician's birthday party in Paris.
Julien Dray invited senior members of his party to the party at a popular disco on the notorious Rue Saint Denis, a street long associated with prostitution.
But Mr Dray did not warn his guests he had invited Mr Strauss-Kahn, who became a toxic figure last year when he was accused of sexual assault in New York and is now under investigation in France over alleged ties to a vice ring.
"He no longer has a role in political life and thus should not be part of a campaign nor in any images that could potentially lead people to believe he's coming back," Mr Hollande said on Sunday.
