Relations have soured between the 2004 Orange Revolution allies
President Viktor Yushchenko of Ukraine has dissolved parliament weeks after the collapse of the country's ruling pro-Western coalition.
Mr Yushchenko announced Ukraine's third general election in less than three years in a pre-recorded speech on TV. The polls will be held on 7 December.
He accused Prime Minister Yulia Tymoshenko of wrecking the government through her "thirst for power".
A Tymoshenko ally accused Mr Yushchenko of violating the constitution.
Ms Tymoshenko and Mr Yushchenko were allies during the 2004 "Orange Revolution" which swept pro-Western forces to power after a discredited presidential election.
Mr Yushchenko's 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 Mr Yushchenko saw as a threat to his presidential powers.
Many analysts believe the prime minister will stand for president in 2010.
"I am convinced, deeply convinced that the democratic coalition was ruined by one thing alone - human ambition," President Yushchenko said in his five-minute speech.
"The ambition of one person. Thirst for power, different values, personal interests taking precedence over national interests."
In an apparent reference to Russian influence, Mr Yushchenko also talked of "external threats".
"In a time of potential external threats, a non-Ukrainian scenario was launched - spineless and, in fact, hostile," he said.
"We see another threat, a tendency brought from abroad, an attempt to break the national and democratic forces. This is a nail which is deliberately being hammered into our body."
The Tymoshenko Bloc, President Yushchenko said, had become "the hostage of its own leaders who would sacrifice everything - language, security, [Ukraine's] European prospects".
In his speech, he said the coalition had collapsed "de jure" on 2 September although the speaker of the parliament, Arseny Yatsenyuk, only declared the coalition officially dissolved on 16 September.
Under the constitution, parties have 30 days to form a new government from the date of its predecessor falling.
Andrei Portnov, deputy chief of the Tymoshenko Bloc in parliament, condemned the president's decision as "unconstitutional and senseless".
"What happened today was 100% provoked by the president, who is the one standing behind the coalition's collapse," he said.
"We will not vote for any bill legalising these anti-constitutional acts of the president."
Viktor Yanukovych, leader of the Party of Regions, described the dissolution of parliament as the "death knell" for Ukraine's current leadership, and said his party was ready fight an early election.