The surprising poll, which is so far out of step with every other election prediction, also claimed that the Tories would lose their majority and cause a hung parliament.
While Mrs May played down YouGov’s findings yesterday, rival pollsters have queried the controversial results.
Tory campaign aide Jim Messina, who helped mastermind President Barack Obama’s election victories, criticised the findings as “yet another stupid poll from YouGov”.
A shock YouGov predicted that Theresa May could lose 20 seats in the House of Commons
The General Election is just days away
Mr Messina appeared so sure that Theresa May will win that he challenged YouGov general manager Ben Leet to a bet over the prediction.
In a message on Twitter, he said: “Spent the day laughing at yet another stupid poll from YouGov. Hey Ben Leet, do you want to bet for charity?”
Another pollster, Andrew Hawkins of ComRes market research, questioned the validity of the poll’s results.
He said: “If voters behave the way they broadly did in 2015 then the Conservatives remain on track for a 100-plus majority.
“That seems, on present assumptions, the most likely outcome.”
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We know we run a risk publishing so much data in the heat of an election campaign
YouGov co-founder Stephan Shakespeare
While Deborah Mattinson, co-founder of polling firm Britain Thinks, and Labour former pollster under Gordon Brown, described the findings as “brave stuff”.
Speaking on last night’s BBC Newsnight, Ms Mattinson added: “The Tories lost a little bit of that message discipline. They allowed themselves to be distracted from their basic Brexit positioning, which I now know they have gone back to. And I think if now they can stick to that then they can coast home and it will not be a problem for them.”
YouGov co-founder Stephan Shakespeare claimed the findings were conducted using the most sophisticated analysis ever deployed and that the prediction was “based on multilevel regression and post-stratification analysis”.
But he added: “We know we run a risk publishing so much data in the heat of an election campaign.”
The projection uses a controversial new method for predicting how voters will switch between parties and how likely they are to vote.
Data from 7,000 voters is extrapolated across the UK’s 650 constituencies to predict who will win each seat.
Using this method means the sample size in any given constituency is likely to be tiny and appears to have thrown up some contested results.
Speaking about YouGov’s findings, Mrs May said: “The only poll that matters is the one that's going to take place on June 8 and then people will have a choice as to who they want to see as leader, who they want to see as Prime Minister taking this country forward in the future, me or Jeremy Corbyn.”