With a week to go before the national election, the poll showed the Conservatives' share of the vote had fallen by four percentage points since last week to 44 percent, while Labour's had increased by three points to 36 percent.
The pound fell sharply after a projection suggested the Conservative Party could fail to win an outright majority in the election on 8 June.
The YouGov survey showed Labour could expect to gain almost 30 seats.
The analysis is based on a complex model and suggests Mrs May's gamble of calling a snap election in the hope of a landslide win could backfire spectacularly. It was trading at $1.2800 early on Wednesday.
Meanwhile, spread betting firm IG said May's performance in the "debate" and the party's social care u-turn had seen clients become "more bullish" on Labour's chances in the June 8 election.
Leaving the European Union would mean "there is no longer a legislative authority over United Kingdom law within the European Union or parliamentary consent for it".
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The government has come under fire in recent weeks for its manifesto pledges, and the introduction of several controversial policies such as removing free school lunches and the so-called "dementia tax", both of which have been received unfavourably among many sections of the electorate.
Meanwhile, on the next Prime Minster market Theresa May is at the longest odds she has been since calling the election at 2/13, but still clearly ahead of Jeremy Corbyn who is at 13/2. YouGov used a model which analysed data from around 50,000 panellists interviewed on their voting intention over the course of a week.
"We are fighting this election to win and we are mounting a fantastic campaign in order to get that message across of how different our society and our politics could be", he said.
YouGov allowed for big variations in the outcome of the election, ranging from as high as 345 seats for the Conservatives, 15 more than their current number, to as low as 274, the pollster's chief executive, Stephan Shakespeare, said.
The puzzle in 2017 is whether the Brexit voters have remained engaged or not, and if so, which party will attract their votes.