Mr Berlusconi admits he is 'no saint'
The publisher of Italy's second largest-selling newspaper is suing Prime Minister Silvio Berlusconi.
Mr Berlusconi described La Repubblica as "subversive", prompting L'Espresso media group to sue for defamation.
L'Espresso also publishes a magazine of the same name, and both publications have led recent investigations into Mr Berlusconi's personal life.
The group also said the PM had discouraged businesses from buying advertising space in its publications.
According to a complaint lodged with a Milan court, the group's lawyers have also accused Mr Berlusconi of abuse of office and of flouting market rules.
Mr Berlusconi's own media empire spans television, newspapers, advertising and film.
He has not yet responded to the allegations.
Earlier this week, the publications released transcripts and audio from what they said was a night Mr Berlusconi, 72, spent with an escort.
The prime minister's lawyer had warned the media against publishing details of the tapes, which he said were "totally fictitious and the product of the imagination".
On Wednesday, in his first public remarks since the audio and transcripts were published, Mr Berlusconi sought to brush off the scandal, which does not appear to have dented his popularity in Italy.
"I am not a saint, you've all understood that," he said.
"I hope those at La Repubblica also understand it."
Patrizia D'Addario told L'Espresso she had made the tapes during a visit to Mr Berlusconi's official Rome residence.
In one conversation, a man can be heard telling a woman to wait for him in "Putin's" bed after having a shower.
Mr Berlusconi - whose personal life has been under scrutiny since his wife filed for divorce in May - has not denied Ms D'Addario attended a party at his home, but insists he did not pay for sex.
The recordings of conversations purportedly between the prime minister and Ms D'Addario were published several weeks after the former model gave them to magistrates investigating Giampaolo Tarantini, a businessman from the southern Italian city of Bari, who is suspected of corruption and abetting prostitution.
Ms D'Addario says she spent the night at the Palazzo Grazioli in Rome
In one exchange, a male voice said to be Mr Berlusconi's can be heard saying: "I'm going to have a shower too... So wait for me in the big bed if you finish first."
A woman's voice, purportedly that of Ms D'Addario, asks: "Which big bed... Putin's?", reportedly a reference to a four-poster bed which Russian Prime Minister Vladimir Putin used during a recent official visit.
Ms D'Addario, 42, says she made recordings of her encounter with Mr Berlusconi "so that nobody could deny I had been there".
Last month, she said she had been paid more than 1,000 euros (£862; $1,420) to attend a party at the Palazzo Grazioli in October, in the company of other women.