French President Nicolas Sarkozy is to file a complaint against a website that claimed Libya's Col Gaddafi had offered to fund his 2007 election campaign.
In six days' time, he faces Socialist Francois Hollande in the second round of the presidential election.
The Mediapart site has published a 2006 document signed by former head of Libyan intelligence Moussa Koussa proposing up to 50m euros in funding.
Mr Sarkozy called it a "crude forgery" and Mr Koussa said it was a fake.
"Do you think that with all that I'd done to Mr Gaddafi, he'd have made me a bank transfer? Why not a signed cheque?" he told France 2 TV on Monday.
The allegation that Col Muammar Gaddafi had offered illegal funding for Mr Sarkozy's 2007 election campaign first surfaced in March 2011 when the late Libyan leader's son, Saif al-Islam, said he was ready to reveal all the details.
At the time, France was spearheading Nato-led efforts to impose a no-fly zone aimed at halting the advance of pro-Gaddafi forces against Libyan rebels.
But this is the first document linked to the allegation, supposedly signed by Mr Koussa and addressed to the head of Col Gaddafi's chief of staff at the time, Bashir Saleh, who ran Libya's sovereign wealth fund.
Mr Saleh said in a statement from his lawyers that the "politically intentioned" allegations were "completely unfounded".
Speaking on French TV, Mr Sarkozy linked the publication of the document clearly to Sunday's election run-off with Mr Hollande and promised to file a complaint by the end of the campaign. "The election campaign doesn't justify everything," he complained.
"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!"
Responding to Mr Sarkozy's threat to take legal action, Mediapart said that the outgoing president was "decidedly opposed to media independence and shows it by his reaction to our latest revelations on the Libyan secrets".
The website added that those in power preferred to insult journalists whose information upset them rather than respond to the questions they posed.
Mr Hollande, who is currently eight points ahead in the latest opinion poll, told Europe 1 radio that it was for the judicial system to take up the matter.
"If it's a fake, well then the website will be condemned," he said. "And if it's not a fake, then at that point there would be some explaining to do."
The Socialist candidate is himself facing an awkward political moment with the apparent re-emergence of former IMF head Dominique Strauss-Kahn, once a favourite for the party's presidential nomination.
Mr Strauss-Kahn, who is under formal investigation for alleged involvement in a vice-ring, was invited to a Socialist MP's party on Saturday to which several figures in the party's presidential campaign were also attending. Mr Hollande later emphasised that Mr Strauss-Kahn "no longer has a role in political life" and was not part of the campaign.
Mr Strauss-Kahn has denied giving an interview published by the British newspaper, the Guardian, in which he is said to have suggested that his arrest in New York last year on charges of sexual assault was manipulated by President Sarkozy for political purposes.
A spokesman said the words were a montage from a forthcoming book by New York journalist Edward Jay Epstein.