Mediapart published Saturday what it claimed to be a copy of a Libyan document showing Tripoli agreed to give Sarkozy's 2007 presidential campaign more than $66 million, Radio France Internationale reported.
Sarkozy, who is running for re-election, denied the allegations.
"It's despicable," he said. "It's a forgery. Mediapart is well used to dishonesty. It's an agency in the service of the left."
The man to whom the document is addressed, Bashir Sal eh, Gadhafi's former chief of staff and head of Libya's sovereign wealth fund, denied receiving the document.
Sarkozy said he will make a "formal complaint" against Mediapart for producing a "fake" document, The Daily Telegraph reported.
"We will file a complaint against Mediapart," he said Monday. "This document is a crude forgery."
The complaint will be filed this week, Sarkozy said, accusing the Web site of smearing him as he goes into the second round of election this week.
Francois Bonnet, the editorial director for Mediapart, has rejected allegations that the document is fake and being used as a smear campaign.
"It's simply grotesque and defamatory," he said.
Sarkozy's opponent, Francois Hollande, said if the justice system determines the document is "a fake, well, the site will be condemned. If it is not a fake, then there will be some explaining to do."