Silvio Berlusconi's increasingly beleaguered government is again in crisis after losing a crucial vote in parliament.

Amid speculation about a possible plot to bring down Italy's 75 year-old prime minister and his shaky rightwing coalition, officials said the government would seek a vote of confidence later this week to stay in office. Berlusconi's chief ally, Umberto Bossi, the leader of the Northern League, said: "I don't know how long it will last."

On Tuesday night, the government failed by two votes to secure approval for the 2010 outturn public accounts. Minutes later, a visibly furious Berlusconi swept out of the chamber of deputies, the lower house, to opposition cries of "resign!".

Commentators said there was no precedent for such a defeat in the 65-year history of the Italian republic. But on both previous occasions when governments had got into parliamentary difficulties over the public accounts, it had ended with the resignation of prime ministers.

What made Tuesday's defeat all the more embarrassing was that neither Bossi nor the finance minister, Giulio Tremonti, took part in the division. Bossi was chatting to journalists in the lobby while Tremonti was racing back from a funeral.

Some commentators sniffed a conspiracy. Though his relations with Bossi have been strained by recent disagreements over economic policy, Tremonti was for years the minister in Berlusconi's party closest to the Northern League. Furthermore, he has been the butt of relentless criticism from the prime minister, both in private and public, for insisting on a policy of fiscal stringency in response to the eurozone crisis.

In a front-page editorial, the daily newspaper Corriere della Sera said the fall of the government was not inevitable. "But there is a palpable risk of governmental paralysis," it said.

Since a rebellion last year by his former lieutenant, Gianfranco Fini, Berlusconi's grip on the lower house has been tenuous. It depends largely on deputies the opposition claims were bought with promises of high office, or even cash – a claim the government denies.

Berlusconi, meanwhile, is beset by problems in the courts. He faces a verdict early next year in a trial in which he is accused of bribing his former offshore legal adviser David Mills. He has pleaded not guilty. And he is a defendant in two other cases, including one in which he denies charges of paying an underage prostitute.