The YouGov model suggested May would lose 20 seats and her 17-seat working majority in the 650-seat British parliament, though other models show May winning a big majority of as much as 142 seats and a Kantar poll showed her lead widening. However the Labour leader has unequivocally ruled out a deal with the SNP and Nicole Sturgeon who infamously swallowed up the Labour majority in Scotland in the 2015 election.
How reliable is the data?
But YouGov acknowledged that models could not produce estimates as accurate as a full-scale poll in each constituency.
The model was based on a voting intention of 41 per cent for the Conservatives and 38 per cent for Labour, a difference of three points, the pollster said.
The study, which entailed thousands of interviews with a panel of participants over a week, was the same approach used by YouGov to gauge sentiment about the referendum on withdrawal from the EU.
The polling figures came in stark contrast to earlier expectations that the Conservatives would increase their majority.
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Prime Minister Theresa May has said the "only poll that matters" is the one on June 8 while Labour's Angela Rayner dismissed the research, insisting Labour is "in it to win it".
A collective "poll of polls" by the Press Association, based on a seven-day rolling average of all published polls, places the Tories on 44%, Labour 35%, the Liberal Democrats on 8%, Ukip on 5% and the Green Party on 2%.
How trusted are the polling companies?
While doubts over polling remain after a failure to predict the UK's 2015 election result, the moves yesterday are emblematic of how the market has been shocked out of its pre-election complacency by mounting evidence that the race is getting tighter.
This time past year David Cameron took one of the most ill-judged gambles a British Prime Minister has ever taken with the European Union referendum - which unlike most constitution-changing referendums did not require a 60% majority and he paid the price for his arrogance. ICM, for example, calculates its figures on an assumption that younger people will be less likely to vote than older, and less affluent people are less likely to vote than the wealthy.
He added: "This has been the general pattern of general elections for an age, and whether you believe our poll findings or those of others will depend on whether or not you think Jeremy Corbyn can actually buck that trend".