That could leave the Tories 16 seats short of the overall majority of 326 needed to govern without the support of other parties.
Sterling fell more than half a percent against the United States dollar to $1.28.
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.More news: Playboy playmate sentenced to probation, community service in body-shaming case
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That would have uncertain consequences for Britain's $2.5 trillion economy, and future government policy on everything from government spending and corporate taxation to bond issuance.
The Labour Party would gain 28 seats to reach 257.
But polls had shown May's rating slipping over the past month and they fell sharply after she set out plans on May 18 to make some elderly people pay a greater share of their care costs, a proposal dubbed the "dementia tax" by opponents.
A hastily-arranged General Election in the United Kingdom - that was seen as a foregone conclusion - has suddenly sprung into life with polls tightening and some market analysts speaking about outcomes that would have been inconceivable just weeks ago. YouGov used a model which analysed data from around 50,000 panellists interviewed on their voting intention over the course of a week.
Last night on the BBC One channel in the United Kingdom they held a live debate with all of the main political parties taking place, however whilst every political party fielded their leader on that debate, the Conservative leader that being Theresa May was not the one in attendance, instead it was left to the Home Secretary Amber Rudd to take the flack who was just recovering from the death of her father! But YouGov has also acknowledged the data could change dramatically before polling day on June 8.
Other projections suggested May would win soundly. The participants returned to their core messages in their final statements, with Mr Corbyn promising that Labour would vote "for the many, not the few" and framing the election as a choice over "whether older people get the dignity they deserve or see their incomes fall". The first round of talks on Brexit in Brussels is scheduled for June 19.