Italian politics has been thrown into a state of renewed uncertainty after Silvio Berlusconi – enraged by his conviction for tax dodging – warned that he was considering bringing down Mario Monti's government and pitching the country into a snap election.

Earlier, he had announced that he would remain in politics "to reform the world of justice".

Speaking at a press conference on Saturday, a visibly irate Berlusconi lashed out at a "judge-ocracy", which he said was running Italy. In a 55-minute monologue, he also turned his rage on Angela Merkel (who, together with the former French president Nicolas Sarkozy, he portrayed as the architect of his removal from office last year) and Monti, whom he depicted as her poodle.

Slipping into the royal "we", the former prime minister declared: "In the next few days, we shall decide with the senior members of my party whether to withdraw confidence from the government or, given the closeness of the end of the legislature, leave it to work out its mandate."

His threat was delivered on the eve of a crucial regional vote in Sicily – the first big test of public opinion in the approach to the general election, which must be held by next spring.

The former prime minister confirmed last week he would not be standing for re-election. But his family's newspaper, Il Giornale, reported on Sunday that – out of range of the microphones – Berlusconi had said the election could be held in January and, that being the case, there would be no time to choose his replacement.

He used the press conference – packed with supporters who periodically broke into applause – to set out what looked very much like an electoral manifesto. His main points included the abolition of a limit on cash payments introduced by the Monti government to curb tax evasion, and the abolition of an unpopular property tax that was re-imposed as part of a drive to eliminate Italy's budget deficit.

On paper, Berlusconi's party, the Freedom People (PdL), could strip Monti and his technocrats of their majority in the upper house of parliament. But it remains to be seen how many of the PdL's lawmakers would follow the party founder's orders.

The PdL has been haemorrhaging support for months now, despite an earlier suggestion from Berlusconi that he would stand again. On Sicily, it faces a stiff challenge from a centre-left alliance and the Five Star Movement, led by the comedian and blogger Beppe Grillo.

Il Giornale reported that the PdL's senior officials, who had not been told in advance of what Berlusconi planned to say, were "in shock". Some were furious, it said.