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.
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 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 on Monday 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.