In his statement, Mr. Cowen said his government would present a four-year plan to reduce the deficit to 3 percent of gross domestic product by 2014, from 32 percent, and preside over the 2011 budget to be voted on Dec. 7.

Passage of the budget, which will call for $8.2 billion in savings, will be the first major hurdle the government faces to ensure receipt of the $100 billion it desperately needs to remain solvent. The Green Party defection came as a shock even to Mr. Cowen, who huddled with its leaders for five hours on Sunday and had no idea they were on the verge of calling for a new election.

Whatever the case, it is now almost certain that a general election will be held early next year.

“The mood in the country is for an election, and the people want a new mandate — that much is clear,” said Joan Burton, deputy leader for the opposition Labour Party.

Ms. Burton, who is also the financial spokeswoman for Labour, said that party leaders would need to see more details on the budget proposed by Mr. Cowen, and that until they did, they were not inclined to recommend it.

But the leader of the Green Party, John Gormley, made clear in a statement that he would not jeopardize the country’s financial bailout by challenging the budget, even though it is likely to call for such harsh measures as a sharp decrease in the minimum wage (currently one of the highest in Europe) and reductions to universal child benefits — payments the country makes to parents with children regardless of their income level.