And Le Monde newspaper says the phone taps have revealed evidence of tampering with the justice system.
It says a senior prosecutor in the country's highest court was feeding Mr Sarkozy confidential information.
Mr Sarkozy's lawyer denies the claims and says the phone taps were illegal.
The BBC's Hugh Schofield in Paris says that the investigators who ordered the taps were looking into allegations, unproven, that Mr Sarkozy had taken illegal payments for his election campaign from late Libyan ruler Muammar Gaddafi.
According to Le Monde, what the investigators discovered from the phone taps was that Mr Sarkozy was getting inside information from the courts about the course of various inquiries into his past.
This information was allegedly being fed from a senior prosecutor at the appeals court whom, Le Monde says, Mr Sarkozy tried to reward with an official post in Monaco.
Mr Sarkozy's lawyer, Thierry Herzog, said on Friday that his client "is probably still being tapped" and denounced what he said was a politically motivated plot against him.
He told AFP news agency: "There was no attempt to pervert the course of justice and in due course this monstrous violation will be shown to have been a political affair."
Mr Sarkozy is planning a political comeback, and our correspondent says that the drip of allegations like this has the potential to do him harm.
It was in 2011 that Col Gaddafi's son, Saif al-Islam, accused Mr Sarkozy of taking millions of his father's money for illegal campaign funding, a claim Mr Sarkozy has strongly denied.
At the time France was spearheading Nato's military campaign in Libya.
Mr Sarkozy, who lost the 2012 presidential election to Francois Hollande, is also under formal investigation over claims he received illegal donations for the 2007 race from France's richest woman, 90-year-old L'Oreal heiress Liliane Bettencourt.
He has denied all the allegations.
Former French President Nicolas Sarkozy has had his phone tapped for the past year on the orders of judges investigating alleged campaign donations from Libya, it is claimed.