Judges ordered the former French president Nicolas Sarkozy's phones to be bugged last April as part of an investigation into allegations that his 2007 election campaign was bankrolled by Libya's then leader, Muammar Gaddafi, it was claimed on Friday.

The phone tapping, believed to be the first of its kind against a French leader, was described by Sarkozy's lawyer as a "monstrous" violation of his legal rights. But transcripts of the taped conversations have reportedly embroiled him in further damaging corruption allegations.

The tapping revelations came after a week in which secret tapes, said to be of conversations between Sarkozy, his wife, Carla Bruni, and advisers were leaked to a satirical magazine.

The claims of phone tapping were described by Nicolas Sarkazy's lawyer as a 'monstrous' violation of his client's legal rights. Photograph: Dominique Charriau/Getty

Le Monde said the new accusations against Sarkozy "have all the ingredients of a state scandal", adding that when judges opened an investigation into claims that his successful 2007 presidential election campaign had financial backing from Gaddafi, they took an unprecedented step.

"Discreetly, the judges took a serious decision: they decided to tap [the phone] of the former president as well as two former interior ministers, Claude Guéant and Brice Hortefeux," said the newspaper.

Somehow Sarkozy got wind of this and became "laconic" in his phone manner, claimed Le Monde. He then obtained a second mobile, which he used to contact his lawyer Thierry Herzog, who had also reportedly bought a second mobile, using a "borrowed identity".

It is not know what the phone taps revealed about Gaddafi, but transcripts of bugged conversations between president and lawyer alerted investigators to how surprisingly "well informed" they were about a separate legal case, said Le Monde.

That case involves an appeal court inquiry into whether the seizure of Sarkozy's diaries by police investigating political corruption was legal. The cour de cassation – France's highest appeal court – is due to rule on this issue on Tuesday.

Le Monde claims investigators established that he and his lawyer were being fed confidential information from the case files by a senior prosecutor, described as a "veteran supporter" of the political right.

As a result, judges have now opened another legal investigation into violation of legal secrets and abuse of influence, which prompted recent police raids on the homes and offices of Herzog and the prosecutor.

On Friday, Herzog denied that either he or his client had tried to sway the investigation and accused investigators of breaching lawyer-client confidentiality over the tapping allegations.

"It's a monumental violation of the right to a [fair] defence. Mr Sarkozy is answerable to the law as others. I am a lawyer as others. I am not asking that either he or I should be treated any differently. We should be treated the same as others," he told Le Monde. He added that he would show that the allegations were "a politically motivated"".

Sarkozy has been dogged by a series of legal investigations and scandals since he lost to François Hollande in 2012 presidential election. Last October, allegations that Sarkozy and his centre-right UMP received campaign donations from France's richest woman, Liliane Bettencourt, were dropped.

On Monday, he and his wife will ask a judge in Paris for an injunction to stop the further publication of transcripts of taped recordings, made by one of Sarkozy's former aides Patrick Buisson. Buisson is also taking legal action claiming the tapes were stolen and used without his permission.