The details have emerged from new documents submitted by prosecutors on Wednesday to a parliamentary committee.
Berardi’s presence at Mr Berlusconi’s villas in Sardinia and Milan has been proven by mobile phone records that an Italian media reported.
Berardi attended a party at Porto Rotondo, in Sardinia, on 21 November 2009, just a few days before her 18th birthday. She also attended a party at his villa at Arcore, near Milan, in December the same year, according to researchers.
Berlusconi assured that the allegations against him are politically motivated; he has refused to attend questioning on the matter and has vowed to punish the magistrates who, he insists, are pursuing a political vendetta against him.