Ukraine's prime minister, Yulia Tymoshenko, last night appeared to have forced a runoff in the race to become the country's new president, closing the gap on her chief rival, Viktor Yanukovich, enough to ensure that the pair will have to face each other again in a deciding vote next month.

Exit polls showed Yanukovich winning the most votes in yesterday's presidential election, but analysts expect Tymoshenko to pick up a higher proportion of second round votes from defeated candidates. They say Yanukovich may struggle to extend his appeal beyond his support base in Russian-speaking eastern Ukraine.

Exit polls showed Yanukovich, 59, a former prime minister who was declared president until the country's Orange revolution of 2004, won 31-38% of the vote. Tymoshenko, 49, who helped to lead the pro-western revolution against his victory and is most popular in the European-leaning west of the country, scored between 25 and 27%.

With only a fraction of votes counted, first official results showed Yanukovich on 38% and Tymoshenko on 25%. Counting continued through the night.

Analysts said Tymoshenko was in a strong position to leapfrog Yanukovich when both candidates meet in a decisive runoff vote on 7 February.

"The eventual size of Yanukovich's lead is key," said Andrew Wilson, senior fellow at the European Council on Foreign Relations. "His strategy was to deliver a knock-out blow in round one. The exit polls show his lead to be not a decisive one."

In third place, with 14.5%, according to one exit poll, was the multi-millionaire former banker Sergey Tigipko, 49, who appeared to be enjoying a late surge. He now appears to be in the position of king- or queen-maker. The votes of his supporters will be crucial in the second round. Tigipko may be persuaded to back Tymoshenko in return for the prime minister's job.

The election took place against the backdrop of a severe economic crisis in Ukraine and amid widespread disillusionment with the country's self-serving political elite. The incumbent president, Viktor Yushchenko, is deeply unpopular and many voters dislike both of the current candidates equally.

Both Yanukovich and Tymoshenko have pledged to reset relations with Russia – reversing the pro-western Yushchenko's hostile policies towards Moscow. At the same time, however, both are broadly committed to Ukraine's integration with Europe, especially in economic matters.

A Yanukovich victory would mean an extraordinary reversal of the dramatic events of 2004, which saw the supreme court overturn his fraudulent victory in the second round of the presidential election, and order another vote – which Yushchenko comfortably won.

In an interview with the Guardian, the deputy prime minister, Hryhoriy Nemyria, claimed that electoral violations occurred ahead of yesterday's vote, and hinted that the contest may be decided not in the ballot box, but in the higher administrative court.

Both Tymoshenko and Yanukovich, in the event of defeat, are likely to accuse each other of fraud and appeal. The court, however, currently has two rival judges – a symbol of the partisan political standoff that has paralysed the government during the past five years.

Nemyria, a member of the Block of Yulia Tymoshenko (ByuT), said that opinion polls "always" underestimated her real support. He predicted that her formidable campaigning and speech-making skills would see her beat the gaffe-prone and "wooden" Yanukovich in the second round.

Nemyria was scathingly dismissive of Yanukovich, who served two jail terms in his youth for robbery and other crimes. "It would be a humiliation if Yanukovich becomes president. Can you imagine portraits of a criminal who was in jail twice hanging in police stations and kindergartens?" he remarked sarcastically.