French President Nicolas Sarkozy "never sent" a text message to his ex-wife Cecilia urging her to come back to him, his lawyer says.
The couple's 11-year marriage ended last October
Thierry Herzog said the website of Le Nouvel Observateur had published a "gross untruth" about Mr Sarkozy.
According to the website, Mr Sarkozy sent a text message saying: "If you come back, I'll call it all off".
It is said to have been sent eight days before he married ex-model Carla Bruni. Mr Sarkozy is now suing the website.
Mr Sarkozy, 53, and Cecilia Ciganer-Albeniz - also a former model - announced their divorce on 18 October.
Mr Sarkozy married Carla Bruni on 2 February this year.
Mr Herzog says a lawsuit has been filed against the nouvelobs.com website for "falsification, use of false documents and possession of stolen goods".
He told the newspaper Le Parisien on Sunday that "this SMS was never sent by the president".
Mr Herzog said he did not think it necessary for police to check Mr Sarkozy's mobile phone messages - a check of Cecilia's would show that she had not received such a message.
On Saturday the editor of Le Nouvel Observateur's website, Airy Routier, stood by the report, saying "I have my sources, I can confirm [it]". He denied any "falsification".
French media say the president's action against a major news weekly is unprecedented in France.
It comes just days after Mr Sarkozy and Ms Bruni won a case against low-cost airline Ryanair for using their picture without consent.
The court in Paris awarded Ms Bruni only 60,000 euros (US$89,000; £45,000) - a fraction of the 500,000 euros she had asked for.
Mr Sarkozy was given the symbolic single euro he had claimed for himself.
The couple's relationship has been front-page news worldwide since it began late last year, after the president's divorce.
The publicity surrounding Mr Sarkozy's private life contrasts sharply with the French tradition of restraint regarding a president's family and love life.
Separately, Mr Sarkozy is currently embroiled in a political row over a decision in his UMP party to drop his mayoral candidate in Neuilly-sur-Seine, a chic Paris suburb that is Mr Sarkozy's home base.
Mr Sarkozy on Monday refused to accept the resignation of David Martinon, his spokesman who is running to be mayor of Neuilly.
Mr Sarkozy's oldest son Jean and two UMP allies earlier announced that they were breaking ranks with Mr Martinon ahead of next month's local elections.