Naples, June 26 - Silvio Berlusconi "heavily" invested in the political campaign of former Senator Sergio De Gregorio, a businessman told investigators probing allegations of bribery by the ex-premier. Prosecutors in Naples want Berlusconi to stand trial for bribing De Gregorio to switch political sides to help defeat a political rival. Businessman Bernardo Martano said that he told investigators that De Gregorio also said Berlusconi was someone that could not be refused. "You never say no," to Berlusconi, leader of the center-right People of Freedom (PdL) party, said Martano, quoting De Gregorio, who also faces criminal trial. De Gregorio has already told prosecutors that he received millions from Berlusconi, convicted earlier this week of having sex with an underaged prostitute and abuse of office. De Gregorio defected from the centre-left during Romano Prodi's 2006-2008 government and eventually joined Berlusconi's party. Berlusconi allegedly paid the turncoat three million euros to switch sides from the centre-left to the centre-right, say prosecutors. The Prodi government fell in 2008 after losing the support of the Senate, leading to new elections that Berlusconi won. De Gregorio originally admitted to receiving two million euros from Berlusconi, which he did not declare for tax purposes, but denied that it was bribe money. In March the Italian daily newspaper Il Messaggero quoted De Gregorio saying he took the money to join Berlusconi's formation with the explicit purpose of bringing down Prodi. Berlusconi has been hit on numerous fronts by legal woes, which he chalks up to left-wing magistrates who allegedly have it out for him for political reasons.