The Conservatives are set for defeat in an upcoming by-election, a party grandee has acknowledged.
Lord Ashcroft, a former deputy chairman of the party, said the Tories would "almost certainly" lose the Corby seat and could face the same result in similar contests with Labour at a general election.
Polling commissioned by the peer gave Labour a 15 point lead to take the marginal seat, with Ed Miliband's party on 52%, the Tories on 37% and the Liberal Democrats on 7%.
The survey of 1,500 voters in the constituency showed Labour was set for a "comfortable" win on a swing which, if repeated nationally, would see Mr Miliband take the keys to No 10 Downing Street at the next general election, Lord Ashcroft said.
Corby's Tory MP Louise Mensch has announced she intends to quit the Commons and move to the United States with her family. The high-profile backbencher, who writes fiction under her maiden name Louise Bagshawe, won the seat from Labour in 2010 with a majority of 1,951.
In an article on the Conservative Home website Lord Ashcroft said: "The Conservative Party will no doubt put up a fight in Corby, but it will have its work cut out."
But he said the polling revealed "mixed" news: "On the one hand the Tories will almost certainly lose a seat which we put considerable energy into winning only two years ago. As things stand, we could expect to lose many similar marginals where Labour are in second place."
He added that previous polling had indicated the Tories faced a struggle to retain marginal seats in fights with Labour.
"Voters are pessimistic about the economy, and are more likely to say the Government is doing badly than it is doing well; few think Conservatives share their values, will do what they say or are on the side of ordinary people."
But, he added: "On the other hand, it is perhaps surprising that the Labour lead is not even bigger in Corby, particularly given the reason the by-election has come about."