A major poll has predicted the Conservative Party will win a Parliamentary majority of 28 seats at the general election, but indicated that the race has tightened in the past few weeks.
But the figure for Boris Johnson's party is a drop of 40 seats from a previous YouGov poll in November that predicted a 68-seat majority.
YouGov said the surprise element of tactical voting and the tightening in the polls meant a hung parliament could not be ruled out.
If the election was held tomorrow, the research suggests the Lib Dems would win 15 seats - three more than in 2017 - while the SNP would gain six for an overall total of 41.
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 and the campaign enters its final full day, with Tory gains off Labour reduced from 44 to 29 since the November 27 findings.
The poll, for Sky News, suggested the race has significantly tightened in the past few weeks.
YouGov said the results mean they cannot rule out a possible hung parliament as an outcome of Thursday's election - or alternatively a solid Tory majority.
In terms of total seats, 339 would be the Conservative's best election performance since Margaret Thatcher was leader in 1987.
New - Our final MRP of the campaign shows a small Conservative majority likely.
Central estimate is a majority of 28.
However at this stage we still cannot rule out a hung parliament. https://t.co/tcd02e3yJP— Chris Curtis (@chriscurtis94)December 10, 2019
But if Labour drop to 231 it would mean the party's worst performance since 1983.
Plaid and the Green Party will see no change from four and one seats won in 2017 respectively.
YouGov has interviewed approximately 100,000 people about their voting intentions in the past seven days.
It said the margin of error could put the final number of Tory seats anywhere between 311 and 367, suggesting a hung Parliament cannot be ruled out.
Sterling fell by around a third of a cent against the US dollar after the news. Financial markets fear a hung parliament would extend the uncertainty over if or when Britain will leave the European Union, which it is currently due to do on January 31,
Chris Curtis, YouGov's Political 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 per cent or less."
The results came after another dramatic day on the campaign trail, when Mr Johnson described Mr Corbyn as a "Hamas-backing, IRA-supporting, anti-Semitism-condoning appeaser of the Kremlin".
He was also accused by the father of London Bridge victim Jack Merritt of seeing last month's terrorist attack as an "opportunity" to score political points instead of as viewing it as a tragedy.
Meanwhile, shadow minister Jonathan Ashworth apologised to party members after being caught on tape giving a damning assessment of Mr Corbyn .
He said the situation facing Labour was "abysmal" because voters "can't stand Corbyn" and think the party has "blocked Brexit" - but later sought to dismiss his remarks as "banter".
He also claimed that the Civil Service machine would "pretty quickly move to safeguard security" if Mr Corbyn entered Number 10.
Mr Ashworth later conceded that the leak made him "look like a right plonker", but said he made the remarks while "joking around" with Tory friend Greig Baker.