Naples, March 5 - Naples prosecutors probing Silvio Berlusconi in relation to a Senator's change of political colours on Tuesday rejected the ex-premier's request to delay appearing before them for questioning. Berlusconi's lawyers said Monday that he could not meet prosecutors on the days they had given him to choose from - Tuesday, Thursday and Saturday - because of his political commitments. They said he was willing to be interviewed on March 15. The prosecutors, however, have decided that the 76-year-old media magnate does not have a "legitimate impediment" to stop him from appearing on the days they had given, judicial sources said Tuesday. Berlusconi's centre-right coalition came second in Italy's inconclusive general election. Last week it emerged that Berlusconi was being probed in Naples for allegedly paying three million euros to Senator Sergio De Gregorio to switch sides from the centre left to the centre right. De Gregorio, who prosecutors say has admitted to receiving bribes, defected from the centre left during Romano Prodi's 2006-2008 government and eventually joined Berlusconi's party. The case is the latest of a series of legal headaches for Berlusconi. He is on trial in Milan over accusations he paid for sex with an underage Moroccan dancer, and is accused of abuse of office in the same case. Berlusconi is also appealing against a one-year conviction he was handed last year for tax fraud on film rights for his Mediaset TV group. The three-time premier has also been indicted over accusations he was involved in the publication of an illegally obtained wiretap.