The billionaire chocolate baron was backed by 44.6 percent of respondents polled on May 14-18 by two respected Ukrainian agencies whose surveys receive EU financial support.

The figure is just short of the 50 percent threshold the 48-year-old needs to break to avoid a June 15 runoff, leaving Ukraine in the hands of an interim leadership that lacks recognition from Russia even as the death toll climbs in clashes across the separatist east.

Nationalist former prime minister Yulia Tymoshenko was running second with 7.5 percent, just ahead of Anatoliy Grytsenko, a former defence minister on 7.0 percent, and Sergiy Tigipko, a tycoon and another former minister with 5.4 percent.

The survey showed Poroshenko easily beating Tymoshenko in the runoff, with 52.2 percent of respondents backing his candidacy.

Only 9.9 percent said they would vote for the ex-premier, with the rest either undecided or saying they had no intention of voting.

The poll pointed to a high turnout, with 60 percent saying they would definitely turn out for the election and another 19 percent suggesting they would probably do so.

The Ukrainian authorities are mobilising more than 55,000 police and 20,000 volunteers to ensure security on polling day but acknowledge there could be problems in ensuring it goes smoothly in the east.

Russia, facing the threat of further Western sanctions should it interfere in the election, has grudgingly backed the ballot without saying outright that it will recognise the winner.