Yulia Tymoshenko said she would only step down as a last resort
Ukraine's parliament has passed a motion of no-confidence in Prime Minister Yulia Tymoshenko's government.
She and her cabinet will now have to resign, while the new president, Viktor Yanukovych, will try to assemble a new coalition in parliament.
If he fails, the country will face snap parliamentary elections.
Despite defeat in recent presidential elections, Mrs Tymoshenko resisted pressure to quit, saying she would only go into opposition as a last resort.
But before the vote on Wednesday she indicated her cabinet would not stay on as a caretaker government.
"If the dismissal of the government is passed today, at that very same moment our government will leave the cabinet. Our political force will cross into the opposition," she said.
Her new goal would be to hold Mr Yanukovych to account, she said.
"We will protect Ukraine from this new calamity that has befallen her," she said.
The motion did pass, gaining the backing of 243 out of the chamber's 450 MPs. It was reported that seven members of Mrs Tymoshenko's bloc voted against her.
On Tuesday the speaker of parliament said Mrs Tymoshenko's coalition had officially collapsed, as it had failed to prove it had a majority.
As well as Mrs Tymoshenko's own bloc, the coalition also included a bloc linked to former President Viktor Yushchenko, and a third led by the speaker of parliament, Volodymyr Lytvyn.
Mr Yanukovych's Party of the Regions has been in opposition, but he wants to bring it into government.
He now has 30 days to form a new coalition, and 60 days to form a government. If he is unable to do so, he will have to call fresh parliamentary elections.
The Party of Regions has named its deputy leader, Mykola Azarov, as its candidate for prime minister. He said he expected coalition talks to be "finalised in the coming days".
The three-way rivalry between Mrs Tymoshenko, Mr Yanukovych and former President Yushchenko has left Ukraine in political deadlock for several years, undermining its ability to deal with a severe economic crisis.
Analysts said as long as Mrs Tymoshenko remained at the head of the government, the stalemate was likely to continue.