ROME — Prime Minister Prodi resigned after he lost his parliamentary majority, ending his government less than two years after he came to power and increasing the chance of early elections.

Mr. Prodi fell short in a confidence vote in the Senate in Rome, with 161 votes against his government and 156 in favor, confirming that his majority had evaporated with the defection of the Udeur Party and its three senators. Mr. Prodi, 68, presented his resignation to President Napolitano.

The president will now begin consulting Italy’s political parties to determine whether they can agree on a new government, possibly with a limited mandate to change the election law, or call an early vote. The current rules, which favor small parties and lead to fractious coalitions, have contributed to Italy having 61 governments since World War II.

“Now it’s time for elections,” an opposition leader, Silvio Berlusconi, said after the government collapsed, speaking in an interview on his Rete4 television channel. “We want a large majority in the Chamber and Senate.”

Mr. Berlusconi, 71, head of Italy’s biggest party, Forza Italia, and has partners lead the ruling parties by nine points in opinion polls, according to polling company SWG Srl. Supporters of Mr. Berlusconi applauded after the vote and members of the National Alliance Party popped champagne corks on the Senate floor. That prompted Senate President Franco Marini to shout: “Get those bottles off the table; this isn’t a restaurant!”