President Francois Hollande has written a letter supporting partner Valerie Trierweiler in her defamation suit against authors of a tell-all biography, lawyers said on Monday, prompting accusations of political interference.

Trierweiler brought the case against the authors of the "La Frondeuse", or "The Rebel", a saga detailing the former journalist's relations with France's political elite including Hollande himself and his ex-partner Segolene Royal.

Lawyers for Trierweiler, who is seeking 80,000 euros in damages, produced the letter in a Paris court as evidence supporting allegations of defamation and invasion of privacy by the two writers and their publishing house, Moment.

In tiny, handwritten script on paper with no presidential markings, Hollande criticises the authors for "pure invention" in a passage concerning him.

The Socialist president was acting as a private citizen when he wrote the letter because the passage in question concerned him directly, an official in his office told Reuters.

Critics accused Hollande of using his political clout to influence the outcome of the case.

"We are witnessing a total confusion of roles in the executive branch whereby the president intervenes in a legal affair just because he wants to," said Valerie Debord, spokeswoman for the centre-right UMP party.

Interior Minister Manuel Valls also wrote to the court to condemn passages in which he is quoted speaking about Trierweiler. His letter is typed on a ministry letterhead.

"They're using a nuclear bomb to smash a fly," Yves Derai, head of the Moment publishing house, told reporters.

The messy love triangle involving Hollande, Trierweiler and Royal - a former Socialist presidential candidate - has filled French media for months. The episode brings yet more unwelcome attention onto Hollande and Trierweiler, who are not married, just as she was trying to keep out of the limelight.

Hollande was forced to call Trierweiler into line this year after she tweeted her support for a candidate competing against Royal during a legislative election in western France.

The episode undermined Hollande's efforts during his campaign to present himself as "Mr Normal", in contrast with what many viewed as the excessively flashy style of former president Nicolas Sarkozy.

Hollande is struggling to reverse a slide in his popularity, according to several polls, linked to high unemployment and to the sense that he exerts insufficient control over his cabinet.