Valerie Trierweiler's libel suit against two unofficial biographers backfired, after it was revealed that her partner Francois Hollande and his Interior Minister had written letters to the court supporting her case.
The leaked letters raised questions about the independence of the judiciary and the separation of powers in the Republic.
Hollande had pledged before his election last May that, as President, he would "allow justice to function in an independent way."
It also played into the hands of the right-wing opposition party, the Union pour un Mouvement Populaire (UMP).
Its leader, Jean-Francois Cope, asked how Hollande could have given "so many moral lessons to [former President Nicolas] Sarkozy while not applying them to himself," the Independent reported.
Trierweiler, 47, launched her libel action against two journalists who in September published a biography entitled 'La Frondeuse' (The Troublemaker).
She accuses them of invading her privacy and is seeking 80,000 euros in damages.
The book notably alleged that she was in a love triangle nine years ago with a prominent right-wing politician, the former economic recovery minister Patrick Devedjian, and with Hollande, the Socialist party leader, when all three were married or in long-term relationships.
Devedjian is also suing the book's authors.
Hollande's letter was handwritten and on plain notepaper. In it, the President dismissed as "inventions" an allegation that he wrote to the UMP grandee Edouard Balladur in 1995, suggesting a rapprochement with the Socialists.
The letter from the Interior Minister, Manuel Valls, was on paper with a ministry letterhead.
Sources close to the President denied that Hollande's intervention was a form of "pressure" on the court, which is to rule on the case on January 28.
They said that it was a "personal testimony" as a private citizen in the case concerning his partner. (ANI)