Ukraine's President, Viktor Yushchenko, has said he will challenge the sacking of the government in the constitutional court.
The Kiev parliament voted earlier to sack the government over a controversial gas deal with Russia.
A no-confidence motion was backed by 250 deputies in the 450-seat parliament, angry over the deal that will force Ukraine to pay nearly twice as much for its gas imports this year.
Mr Yushchenko, who is on a trip to Kazakhstan, said he believed the vote to oust his government was unconstitutional.
Prime Minister Yuri Yekhanurov, who will remain as acting premier until Mr Yushchenko names a replacement, said he would do everything he could to prevent any impact on the gas deal with Russia which followed acrimonious negotiations.
Moscow cut off gas to Ukraine for two days at the start of the year, also hitting supplies to customers in the West.
The vote is the latest blow to Mr Yushchenko, who came to power on the back of popular protests but who has spent his first year in office battling poor economic growth and corruption scandals.
And it comes ahead of a parliamentary election in March when his supporters will struggle to beat off a challenge by the opposition.
