Labour would scoop 93 seats from the Conservatives and take the keys to No 10 if a general election was held tomorrow, according to a poll of marginal constituencies.
Overall, Labour would gain 109 seats, taking them to a total of 367 MPs and giving them a majority of 84, the study found.
The research, carried out by Lord Ashcroft, who has previously donated millions to Tory coffers, found there would be an 8% swing to the Opposition in the most closely contested seats.
It comes amid claims that a canvass of Tory activists found just 7% believed Prime Minister David Cameron will secure victory in 2015 and is likely to fuel unease among already restive party members.
Nearly 20,000 voters in 213 constituencies were polled for the study which found Labour would win 93 of the 109 most marginal Tory seats, with the biggest swing to the Opposition in the Thames Estuary and the Midlands.
Based on the research, the Lib Dems would lose 17 constituencies in England and Wales to their coalition colleagues and 13 to Labour.
Lord Ashcroft announced the findings at the Victory 2015 conference being staged by website conservativehome.
He said: "I don't want to see a Labour majority of four, let alone 84, but I hope this puts the challenge into some sort of perspective. We have a long way to go to hold onto the seats we gained last time, let alone pick up many more. But things are slightly less grim than the headline polls suggest, and we have everything to play for."
Tory minister Nick Boles said that the party had "screwed up" in the Eastleigh by-election but warned that it must not swing to the right after the drubbing. He suggested the party failed to offer voters any hope and had repeated the same mistakes it has been making for more than a decade.
The comments come as The Times reported that it had seen details of a canvass of Tory supporters that showed just 7% believed the Prime Minister will win an overall majority at the next general election, while three quarters expect Labour to be in power.