The Tories are facing a crushing defeat in the Corby by-election next month, according to a new poll by a Conservative grandee.
Polling by former deputy party chairman Lord Ashcroft puts Labour 22 points ahead with just weeks to go before the vote, due on November 15.
Retaking the seat by the projected 54% to 32% margin would represent a 13-point swing back to Labour - enough for a significant Commons majority, if repeated at the general election.
But there was some hope that the Tories could still regain some ground by 2015 because the results showed Labour is still not trusted to run the economy.
The Corby and East Northamptonshire by-election was forced by Louise Mensch's decision to quit and move to the US with her family. She only won in 2010 by a 1,951 majority.
Writs are also being moved in the Commons for by-elections in Manchester Central and Cardiff South and Penarth on the same day, to coincide with the election of local police commissioners.
These seats were vacated by Labour MPs Tony Lloyd and Alun Michael so that they can contest the first directly-elected police oversight positions.
Lord Ashcroft's previous opinion poll in Corby put Labour ahead by 15 points. He said the latest findings show the advantage is now "entrenched".
Half of those who intend to switch their support away from the Tories say it is because they are "not happy with what the Conservatives are doing in government", the survey suggested.
More than a quarter of defectors say they will "almost certainly or definitely" not switch back in the 2015 general election, though more than half have not ruled out the prospect.
Only a quarter of those polled were satisfied with Prime Minister David Cameron's performance. However a quarter of the 54% planning to back Labour's candidate would rather see the Tory leader in Number 10 than Ed Miliband.
Despite continued pessimism about the economic recovery, the coalition is still trusted more on the economy - at 48% to 33% for Mr Miliband and his shadow chancellor Ed Balls.
Lord Ashcroft, writing on the ConservativeHome website, said the Tories may already have given up and decided to focus on future campaigns.
He said: "The 13-point swing to Labour is not on the same scale as those of Crewe & Nantwich in 2008 or Wirral South in 1997 - but it means a comfortable victory for the opposition and would mean a significant overall majority for Ed Miliband if repeated at a general election."
By-elections in Middlesbrough and Croydon North, following the recent deaths of Labour MPs Sir Stuart Bell and Malcolm Wicks, are expected to be held on a different date.