Meanwhile a separate YouGov model based on different data estimated the Conservatives would win 317 seats, nine short of an overall majority of 326 seats.
In a hectic campaign which was suspended after a suicide bombing last week, pollsters, who universally got it wrong before the last vote in 2015, have offered a vast range for the result of the election: From May losing her majority to a landslide victory for her Conservatives of more than 100 seats.
"From the pollsters' point of view this is an experimental election. We all got it wrong in 2015 and we are all trying different methods to get it right this year," said Anthony Wells, a research director at YouGov.
Betting that she would win a strong majority, May called the snap election to strengthen her position at home as she embarked on complicated Brexit negotiations with 27 other members of the European Union.
But if she fails to beat the 12-seat majority her predecessor David Cameron won in 2015, her electoral gamble will have failed and her authority will be seriously undermined.
If May failed to win an overall majority, she would be forced to strike a deal with another party to continue governing either as a coalition or a minority government.
That would have uncertain consequences for Britain's US$2.5 trillion ($3.5t) economy, and future government policy on everything from government spending and corporate taxation to bond issuance.
YouGov said May was still the most favoured choice for prime minister, though her 43 per cent rating is the lowest it has ever been. Labour leader Jeremy Corbyn is on his highest ever rating of 30 per cent.
Corbyn, a 68-year-old peace campaigner, has been pulling in big crowds at rallies across the country despite warnings from opponents in his own party that he is leading Labour to its worst defeat ever.
May, who won the top job in the political chaos following the shock June 23 Brexit vote, used a speech yesterday in northern England to pitch her vision of Brexit.
"Set free from the shackles of EU control, we will be a great, global trading nation once again bringing new jobs and new opportunities for ordinary working families here at home," said May, who backed the "Remain" campaign for last year's referendum on EU membership.
May was taunted by other party leaders on Thursday for not attending a televised debate with them. Instead, May sent her Interior Minister, Amber Rudd, who dismissed the leaders as members of a "coalition of chaos".
The Conservatives have made an official complaint to the BBC after the debate about the "biased" audience in the leaders' debate and warned there must be no repeat when May and Corbyn appear on a Question Time special.
May had said she was too busy "thinking about Brexit negotiations" and "meeting voters" to take part in the debate.