The constituency-by-constituency estimate for The Times by YouGov indicates the Conservative Party could lose 20 seats and see its majority wiped out, while Jeremy Corbyn’s Labour may gain 28 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.
YouGov’s analysis puts the Tories on 310 seats, down from the 330 they went into the election with, and 16 short of a majority.
Labour would get 257 seats, up from 229, the Liberal Democrats 10, up from the nine Tim Farron’s party held when the election was called, the SNP 50, the Greens one and Plaid Cymru three.
Those figures are from the model’s central estimate, which acknowledges a large range of variation.
A good night for the Tories could see Mrs May’s party pick up 345 seats - a net gain of 15 but still well short of the kind of majority she would have hoped to secure following the decision to go to the country.
A bad night could see the Conservatives plummet to 274 seats.
YouGov’s model draws on the data collected from around 50,000 panellists quizzed on their voting intention over the course of a week and uses a recently-developed technique called multilevel regression and post-stratification (MRP).
The pollster’s MRP model is based on the fact that people with similar characteristics tend to vote similarly, but not identically, regardless of where they live.
During the EU referendum campaign it consistently showed that more voters favoured Leave than Remain.
But YouGov acknowledged that models could not produce estimates as accurate as a full-scale poll in each constituency.
YouGov chief executive Stephan Shakespeare said the data could change dramatically between now and June 8.
“The data suggests that there is churn on all fronts, with the Conservatives, Labour and the Liberal Democrats both likely to both lose and gain seats,” he wrote in The Times.
“Based on the model’s current estimates, some seats are likely to change hands along EU referendum dividing lines.
“This is just a snapshot based on data from the past seven days and people can and do change their minds in the closing days of a general election campaign.
“Furthermore, it would not take a slight fall in Labour’s share and a slight increase in the Conservatives’ to see Theresa May returning to No 10 with a healthy majority.”