Relations have soured between the 2004 Orange Revolution allies
Ukraine's ruling pro-Western coalition has officially collapsed, the speaker of the Ukrainian parliament says.
President Viktor Yushchenko has been involved in a long-running dispute with Prime Minister Yulia Tymoshenko.
The president's Our Ukraine bloc left the coalition earlier this month. Parliament now has 30 days to try to form a new ruling coalition.
If those efforts fail, Mr Yushchenko can dissolve parliament and call a snap election.
The Our Ukraine party pulled out of the coalition on 3 September after the Tymoshenko Bloc sided with the pro-Moscow opposition Party of Regions to pass several laws that Mr Yushchenko saw as a threat to his presidential powers.
"I officially declare the coalition of democratic forces... in Ukraine's parliament dissolved," parliament speaker Arseny Yatsenyuk announced on Tuesday.
"This has been long expected, but for me it is extremely sad," he told the chamber.
"I would not call this a political apocalypse, though it is true that it is another challenge of democracy. I hope we can overcome it."
Mr Yushchenko and Ms Tymoshenko led the 2004 Orange Revolution, which overturned the fraudulent presidential election victory of a pro-Moscow candidate, Viktor Yanukovych. But since then the two former allies have become bitter rivals, vying for power ahead of the 2010 presidential election.
Ms Tymoshenko leads the second largest group in the 450-seat assembly, after Mr Yanukovych's Party of the Regions.
Ukraine's Vice Prime Minister Hrihoriy Nemyria, a member of Ms Tymoshenko's bloc, denied the party was trying to form a coalition with Mr Yanukovych's party.
"Our priority is to re-establish the Orange coalition... The worst case scenario would be an early election in the mid winter," he told the BBC's HARDtalk programme.
Ms Tymoshenko dismissed the breakdown as "a storm in a teacup".
"Let me assure you that the government is going to work for a long time and successfully too, regardless of these storms," she said.
Opposition leader Mr Yanukovych said any new coalition should include his Party of the Regions.
"Any configuration leaving out the country's biggest political forces is doomed to fail," he said.
The HARDtalk interview with Hrihoriy Nemyria will be broadcast on Wednesday 17th September 2008 at the following times on BBC World News at 0330, 0830, 1430, 2030 and 2330 GMT and on the News Channel at 0430 and 2330