Leading Ukrainian politician Vitaly Klitschko has pulled out of presidential elections planned in May.
The former boxing world champion said instead he would back tycoon Petro Poroshenko.
Both men played a key role in months of street protests that led to the ousting of President Viktor Yanukovych.
Klitschko’s withdrawal means the race is likely to be between Poroshenko and former Prime Minister Yulia Tymoshenko.
Several former supporters of the ex-president have also announced they will run. The Party of Regions, to which Yanukovych belonged, on Saturday backed former Kharkiv governor Mykhailo Dobkin to run for the presidency.
The elections on May 25 are seen as a crucial step in leading Ukraine out of the country’s deepest political crisis since its independence in 1991. An interim administration is currently in place in Kiev, led by acting President Olexander Turchynov and Prime Minister Arseniy Yatsenyuk.
Russia says the current authorities in Kiev came to power in a coup and are, therefore, illegitimate.
Ukraine – backed by the West – denies the claim.
Moscow formally annexed Crimea after the predominantly ethnic Russian region held a referendum earlier this month which backed joining Russia.
Kiev and the United Nations General Assembly have condemned the vote as “illegal”.
In response to Moscow’s move, the US and its allies have imposed sanctions on members of Russian President Vladimir Putin’s inner circle, and threatened to take action to target the Russian economy.
On Saturday, Russian Foreign Minister Sergei Lavrov said Moscow had no intention of sending troops into the mainland Ukraine, despite a massive build-up of forces near the border.