Former French President Nicolas Sarkozy and his wife are to file a legal complaint over secret recordings made of them by an adviser during the 2012 election campaign, lawyers for the couple said.

Revelations that Patrick Buisson, part of Mr Sarkozy's inner circle, recorded hours of talks with the conservative leader and his entourage have caused uproar in the opposition UMP party weeks before local elections.

Mr Sarkozy is expected by many to contest the 2017 presidential election after his defeat by Francois Hollande two years ago.

"Mr Nicolas Sarkozy and Ms Carla Bruni cannot accept that remarks made in private were recorded and published without their consent," lawyers Thierry Herzog and Richard Malka said in a statement.

"(They) have decided to prosecute, through an emergency proceeding soon to be filed with the Paris Grand Instance Court, the recording and publication of their conversations," they said.

"Protecting the secrecy of private conversations is one of the founding principles of a democratic society."

The potential for the affair to damage Mr Sarkozy grew after Marine Le Pen, leader of the anti-immigrant National Front (FN), said that Mr Buisson had a secret meeting with her father Jean-Marie, then FN leader, during the 2007 presidential vote.

Excerpts of the tapes were published this week in satirical weekly Le Canard Enchaine and on the right-wing news site Atlantico.

Mr Buisson's lawyer, Gilles-William Goldnadel, has confirmed that the recordings are authentic. He said his client had made them for historical research purposes and had not intended that they be made public.

Breach of privacy in France carries a penalty of up to one year in prison and €45,000 in fines.

In the published excerpts Mr Sarkozy is heard discussing his electoral strategy and a 2011 cabinet reshuffle, while Ms Bruni is recorded joking about how she had to put her modelling career on ice while she was France's first lady.

"I thought I was marrying a guy with a salary, I had big contracts and now nothing," she is heard saying, adding that if Mr Sarkozy went on to lose the election she at least could re-activate her career and start selling anti-wrinkle cream.

French media have said dozens of hours of further material could emerge from recordings that date back to 2011.

Mr Goldnadel said the recordings, which Mr Buisson made using a device in his pocket and later uploaded to his personal computer, had been stolen from Mr Buisson and that he would file a complaint for theft against "persons unknown".

"There will be an investigation to determine exactly how these recordings were taken," Mr Goldnadel told a French television station. He added that his client thought he knew who had stolen them, but did not give names.             

Ms Le Pen said Mr Buisson, former editor of the extreme-right weekly Minute, had asked to meet her father after he won 10.4% of the vote in the first round of the 2007 election that ultimately brought Mr Sarkozy to power.

"After Jean-Marie Le Pen was knocked out of the race, I assume that Nicolas Sarkozy wanted to take the temperature, to see whether he might endorse him, which would not have been so scandalous," she said, adding she had been present at the lunch too.

It was the first time direct talks between the entourage of the relative conservative Mr Sarkozy and the far-right Front have been disclosed.

Mr Sarkozy publicly denounced the Front while seeking to attract far-right voters by hammering home his opposition to immigration, support for law and order and attachment to French national identity.

Neither Mr Buisson nor his lawyer responded immediately to a request for comment.