Ukrainian President Viktor Yushchenko says his political allies can trigger a snap poll if parliament fails to back a law calling for a September election.
Ukraine's leaders have been involved in a bitter power struggle
He said his allies would resign their parliamentary seats if the law was not passed, forcing the snap poll.
He earlier extended until Thursday evening a deadline for parliament to finish debating the election law.
Mr Yushchenko and his rival, PM Viktor Yanukovych, have agreed on the September election to end a stalemate.
Their rivalry, dating back to 2004, has created a political crisis in Ukraine.
Speaking during a day-long visit to Croatia, Mr Yushchenko said his allies would give up their seats in parliament if members did not approve the law within the new deadline.
"So the election will be held automatically within 60 days - this is a clause in the constitution," Mr Yushchenko is quoted by the Reuters news agency as saying.
Parliament had been asked to pass the measures by the end of Wednesday, but failed to do so in time, prompting Mr Yushchenko to extend the deadline by a day.
The latest delay in parliament is reported to have been largely due to objections from Mr Yanukovych's supporters to the president's demand that MPs be prevented from changing parties once elected.
Mr Yushchenko reached a deal on Sunday with his rival, Prime Minister Viktor Yanukovych, to hold an early poll in September and thereby end months of crisis.
Mr Yushchenko dissolved parliament in April, accusing Mr Yanukovych of trying to usurp his power.
MPs are considering laws on election financing, electoral commission reform and the former Soviet republic's bid to join the World Trade Organisation (WTO).
The election deal was reached after a marathon 12-hour session of talks between the two leaders on Sunday.
Mr Yushchenko became president in January 2005 following the pro-democracy Orange Revolution, which overturned a rigged victory for Mr Yanukovych.
But Mr Yushchenko was forced to accept his rival as prime minister after his allies failed to win a majority in the March 2006 parliamentary election, and the two men have repeatedly clashed.
The president favours closer ties with the West, while the prime minister is seen as more pro-Russian.