France's opposition on Tuesday called for the resignation of the head of the country's domestic intelligence agency, a friend of President Nicolas Sarkozy, after he was charged over spying on a journalist.

DCRI spy agency boss Bernard Squarcini was charged Monday with violating confidential correspondence, illegally collecting data and violating the confidentiality of sources of a journalist with the daily Le Monde.

Under French law, preliminary charges are brought to allow magistrates to continue investigations before possibly sending the case to trial.

Investigating magistrate Sylvie Zimmerman is due to interview France's top policeman Frederic Pechenard, also close to right-wing leader Sarkozy, over the same case, and he could also face charges.

Police allegedly obtained the journalist's mobile telephone logs in 2010 in order to identify the source in a complex illegal political financing scandal linked to L'Oreal heiress and billionaire Liliane Bettencourt.

The scandal began as a dispute between the heiress and her daughter but soon evolved into allegations that Bettencourt had been handing bundles of cash to politicians, including Sarkozy's 2007 presidential campaign manager.

A judge on Monday ordered that Bettencourt's estranged daughter take over her affairs because of declining mental health, while the manager, Eric Woerth, lost his job as employment minister last year as a result of the allegations.

"I'm surprised the interior minister hasn't yet called for Mr Squarcini to resign, because what's happened is serious," the Socialists' candidate in next year's presidential election, Francois Hollande, said late Monday.

"This error must be punished immediately," he said.

Green Party leader Eva Joly on Tuesday said that Squarcini should resign immediately.

"I find it absolutely incredible that France's counter-intelligence chief could have used the state's means to spy on a journalist. It's absolutely enormous," she said.

But the spy chief has the support of his superiors, including Interior Minister Claude Gueant, who is also close to an increasingly scandal-mired Sarkozy.

Gueant has said that the charges against Squarcini do not "prevent him fully exercising his duties."

The ruling right has also been at pains to stress Squarcini's presumption of innocence.

Squarcini's lawyer, Patrick Maisonneuve, said the journalist "was not targeted by the technical checks" but simply caught up in steps taken to identify a mole at the justice ministry, who was subsequently fired.