Ireland's Green Party quits government of Brian Cowen
The Greens' announcement wipes out the ruling coalition's two-seat majority and puts into question the passage of a vital finance bill.
It also follows the decision on Saturday by PM Brian Cowen to quit as leader of his Fianna Fail party but to stay on as prime minister.
Opponents said this was "farcical".
Fianna Fail had urged the Greens to stay in government to ensure the financial bill - which is needed as part of Ireland's international bail-out package - was passed before the election.
Ireland was forced to accept the 85bn euro ($113bn; £72bn) EU and IMF bail-out in November last year.
When asked whether he would now resign immediately, Mr Cowen said getting the finance bill through was the main issue.
'Persistent doubts'
The Green Party will join the opposition benches immediately.
But it said it would still support the finance bill and hoped Fianna Fail would fast-track the legislation so it could be approved speedily.
Green Party leader John Gormley, speaking after a meeting in a Dublin hotel, said: "For a very long time we have stood back in the hope that Fianna Fail could resolve persistent doubts about their party leadership.
"A definitive resolution of this has not yet been possible and our patience has reached an end. Because of these continuing doubts, the lack of communication and the breakdown in trust, we have decided that we can no longer continue in government.
"We will remain true to our promise to support the finance bill from the opposition benches."
After the Greens' announcement, Mr Cowen said: "The important thing now is to have an orderly completion of the finance bill in the interests of the country and then obviously we move to a dissolution of the [parliament] and a general election."
The loss of two Green Party cabinet ministers means Mr Cowen now only has seven of 15, the minimum constitutionally allowed.
The focus next week will fall on the finance bill and a no-confidence motion put forward by the opposition for Tuesday.
If Mr Cowen lost the motion he would be obliged to resign and call an election within four weeks.
Correspondents say he might try to persuade the opposition to hold off on that vote to allow for debate and passage of the finance bill, which is designed to cut the government's deficit.
The Greens want all-party talks on Monday to rush the bill through as soon as possible.
But the BBC's Ireland correspondent Mark Simpson says its passage is not assured, given the country's political system is in a state of chaos and filled with bitterness.
Mr Cowen's assertion on Sunday that it is "not possible to deal with it in a week" is unlikely to please the opposition, Kazinform refers to BBC News.
To read more see www.bbc.co.uk .