Boris Johnson remains on course for a Conservative majority of 28 seats – but the Prime Minister has lost ground in the past two weeks and a hung parliament could still be on the cards, according to YouGov.
The polling company, having analysed 105,000 voter interviews between December 4 and 10, predicts the Tories would win 339 seats, giving the governing party a majority of 28.
The result is down on the 68-seat majority that the same YouGov MRP exercise predicted Mr Johnson would end up with only two weeks ago.
YouGov said the surprise element of tactical voting and the tightening in the polls meant a hung parliament could not be ruled out.
The estimated result, if it materialises, would still give the PM the largest Tory majority since the 1980s, topping John Major's 21-seat margin of Commons control in 1992.
For Labour, the prediction of winning 231 seats would be its worst result for more than 30 years, said YouGov.
Jeremy Corbyn's party would fall from 262 in 2017 to 231, according to the newly-published research.
The Tories are predicted to take 43% of the vote and Labour 34%. The same YouGov MRP modelling correctly called 93% of seats in 2017.
If the YouGov predictions bear out, the SNP would win 41 seats and Liberal Democrats 15.
Nigel Farage's Brexit Party is predicted to be winless after the General Election on Thursday.
The result will be a worry to Mr Johnson as he prepares for a final blitz across the country as the campaign enters its final full day, with Tory gains off Labour reduced from 44 to 29 since the November 27 findings.
There are signs that Labour is "patching the cracks" in its so-called "red wall" of seats across the North and the Midlands, say YouGov – but seats such as Don Valley, Bishop Auckland, Wrexham and Great Grimsby are still predicted to go blue.
The seats of Chipping Barnet – Environment Secretary Theresa Villiers' seat – and Putney look to be swinging to Labour, say the pollsters.
Jo Swinson's Lib Dems are on course to win in Richmond Park, St Albans and Winchester, while also likely to win back ex-leader Nick Cleggs' Sheffield Hallam seat.
According to the poll, Plaid Cymru and the Green Party will see no change from four and one seats won in 2017 respectively.
Chris Curtis, YouGov's poitical research manager, said: "Our latest and final poll shows that a small Conservative majority is likely, with the Tories taking 22 more seats than in 2017 and Labour losing 31.
"This would be the best and worst results respectively for each party since the 80s.
"But the margins are extremely tight and small swings in a small number of seats, perhaps from tactical voting and a continuation of Labour's recent upward trend, means we can't currently rule out a hung parliament.
"As things currently stand, there are 85 seats with a margin of error of 5% or less."
YouGov's model draws on the data collected from more than 100,000 panellists questioned on their voting intention over the course of the last seven days, and uses a recently-developed technique called multilevel regression and post-stratification (MRP).
The data extrapolation takes national polls and then applies it to all 650 UK constituencies by using local factors and economics to inform which party is most likely to win the seat.
YouGov stress that the poll cannot read tactical voting efforts, a caveat that adds to the margin of error involved.