The scandal came to light last month when the French media published transcripts of conversations between Bettencourt and De Maistre that were secretly recorded by a former butler in the heiress's household.

The tapes reportedly revealed that Bettencourt plotted to evade taxes, hiding some $103m in Swiss bank accounts while giving big donations to friends in Sarkozy's ruling UMP party.

'Surrounded by predators'

The four men were expected to be held for 48 hours, after which police must decide whether to press charges.

A spokeswoman for the public prosecutor said they were being questioned "about the contents of these secret recordings".

Banier, a society photographer, is already facing trial charged with abusing 87-year-old Bettencourt's frailty to obtain gifts estimated at up to $1.28bn.

Francoise Bettencourt-Meyers, Bettencourt's daughter, has asked a judge to declare her mother incompetent to manage her $20bn fortune after she showered Banier with expensive gifts.

"This is an important step because for months we have been saying that Liliane Bettencourt is surrounded by predators," Olivier Metzner, the lawyer representing the daughter, said.

"We consider that she is not acting responsibly because she is being steered in a direction," he said.

Bettencourt, the main shareholder in cosmetics giant L'Oreal, called in a statement on Wednesday for an audit into firms managing her fortune to show she was mentally competent.

The scandal has drawn in Sarkozy after Bettencourt's former accountant told police that the billionairess had contributed $193,000 to his campaign in 2007 through his fundraiser and current Labour Minister Eric Woerth.

Woerth, who has resigned as party fundraiser, denied the allegations and Sarkozy has suggested they are part of a smear campaign.

The scandal comes at a delicate time for Sarkozy who is struggling with record-low approval ratings and seeking to push through an overhaul of the pensions system in the face of strong union opposition.