Berlusconi has denied corruption in the Naples case. One of his lawyers, Michele Cerabona, insisted that the facts indicate there was "no bribery." Lavitola's defense has also denied the charge.
De Gregorio, in a telephone interview with Sky, said he had told prosecutors what he knew about the alleged payment and added that he later expressed his regrets to Prodi. In collaborating with prosecutors, in "telling the truth, I lifted a weight" off my conscience, De Gregorio said.
Prosecutors investigated allegations that Berlusconi, a billionaire media mogul, paid De Gregorio 3 million euros ($4.1 million) under the table to defect to his party, helping to bring down Prodi's ruling coalition. Prodi was Berlusconi's most formidable political challenger at the time.
Earlier this month, an appeals court in Milan handed Berlusconi a two-year political ban as part of his punishment for a tax fraud conviction. Separately, Berlusconi was already facing the loss of his Senate seat because of the conviction, which was upheld by Italy's highest criminal court during the summer.
And he risks a lifetime political ban and a seven-year prison sentence if appeals courts uphold a conviction in Milan for having paid a minor for sex and forcing officials to cover it up.
Berlusconi has blamed his many judicial woes on magistrates he contends side with the political left.