The Tories are set for crushing defeat in the Corby by-election and may have given up full-scale campaigning to retain the seat, a party grandee has warned.
Polling by former deputy party chairman Lord Ashcroft found Labour had soared into a massive 22-point lead because voters want to punish David Cameron's Government.
Retaking the seat by the projected 54% to 32% margin would represent a 13-point swing back to Labour, enough for a significant overall Commons majority if repeated at the general election.
Former MP Louise Mensch secured a 1,951 majority in 2010
But the latest survey of 1,503 voters in the constituency suggested there remained hope for the Tories to claw back ground by 2015 as Labour remains untrusted to run the economy.
The by-election, forced by the decision of Louise Mensch - who secured a 1,951 majority in 2010 - to quit the Commons and move to the US with her family, will be formally set today for November 15.
Writs will also be moved in the Commons for by-elections on that day in Manchester Central and Cardiff South and Penarth to coincide with the elections of local police commissioners.
The latter two Commons seats were vacated yesterday by Labour MPs Tony Lloyd and Alun Michael to contest the first-such directly-elected police oversight positions in England and Wales.
Lord Ashcroft's previous opinion poll in Corby showed only a 15-point lead for Labour and he said the findings showed its advantage was now "entrenched".
Half of those who intend to switch their support away from the party 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.
Despite only a quarter of those polled saying they were satisfied with Mr Cameron's performance, his party will draw some comfort from Labour's continued struggle to take more of an advantage.
A quarter of the 54% planning to back Labour's local candidate Andy Sawford would rather see the Tory leader in Number 10 than Ed Miliband, the poll suggested.
And despite continued pessimism about the economy - 55% expecting no improvement or worse in the next three to four years - Labour is still struggling to convince the public of its competence.
Mr Cameron, Chancellor George Osborne and Deputy Prime Minister Nick Clegg are favoured by 48% to 33% to run the economy over Mr Miliband and his shadow chancellor Ed Balls.
That includes three quarters of 2010 Tory voters now intending to put their cross elsewhere.
The advantage comes in spite of more people believing Labour would secure growth and jobs than the coalition - which is more trusted to reduce debt and the national deficit.
Writing on the ConservativeHome website about the findings, Lord Ashcroft said Labour was also "winning the ground war" - ahead on leafleting, direct mail and canvassing.
That could be due to finding it "hard to recruit volunteers to defend a seat they did so much to win only two years ago" or that party HQ had effectively conceded defeat, he indicated.
"CCHQ may have decided to conserve resources for future campaigns it sees more hope of winning.
"Unfortunately it looks as though Christine Emmett, the excellent Conservative candidate I wrote about meeting at conference in Birmingham, is going to do less well than she deserves."
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 at a later date not on November 15.