La Repubblica daily and L'Espresso weekly websites posted tapes of conversations purportedly between Berlusconi and Patrizia D'Addario, an escort who says she and others were paid to attend parties at Berlusconi's residence in Rome.
Berlusconi has not denied that the woman went to his home but has said he did not know she was an escort.
One conversation posted on the websites was between D'Addario and Giampaolo Tarantini, a southern Italian businessman who is under investigation by magistrates on suspicion of corruption and abetting prostitution.
D'Addario says she made the tapes on her cellphone during her visits to the prime minister's Rome residence or while she was involved in telephone conversations, one with Berlusconi.
Daniele Capezzone, spokesman for Berlusconi's People of Freedom party, called the posting of the conversations "pathetic." Government minister Gianfranco Rotondi said the leftist media wanted to "intimidate" the government by using "the violation of every ethic" of journalism.
D'Addario, 42, has given the tapes to magistrates investigating the case against Tarantini.
Berlusconi, who is already dogged by a messy public divorce and reports of cavorting with underage girls, has in the past called the accusations by the newspapers "trash and falsehoods."
But L'Espresso and La Repubblica, both left-leaning and both owned by the same publishing house, said the tapes proved "what D'Addario has been saying about the prime minister is true."
The scandals have given the opposition a rare chance to land a blow on Berlusconi, who dominates the political landscape and remains popular despite the economic crisis. He said a recent poll gave his government a 57 percent approval rating.
The current uproar over Berlusconi's private life came nearly two months after Italy was transfixed by his friendship with an 18-year-old aspiring model. He said he had no sexual relations with her.
Berlusconi's lawyer said last month that even if D'Addario's allegations were true, Berlusconi could not control who his male guests brought to the prime minister's residence and he would have been at the very most an "unwitting subject."
La Repubblica, Italy's second largest mainstream daily, has been leading the charge against Berlusconi and breaking most of the stories on his private life, provoking the wrath of the premier and his aides.
Every day since May 14 it has published 10 questions it says he should answer to clear up the controversy. One of the questions asks if he can assure the country that his acquaintances could not blackmail him.
(Editing by Ralph Boulton)