The UK Independence Party has surged to a record poll rating - knocking the Liberal Democrats into fourth place.
The ComRes poll for The Independent on Sunday and the Sunday Mirror put Ukip on 14% - a six-point gain on last month and the party's highest rating in a ComRes poll.
The findings will alarm Tory MPs who fear the party is haemorrhaging support over issues such as Europe, the economy and its support for gay marriage.
The poll put the Conservatives on 28%, down three points from a month ago and 11 points behind Labour, who are down four on 39%.
Support for the Lib Dems is down to single figures at 9%, a one-point fall on the month.
Andrew Hawkins, chairman of ComRes, said "there is good evidence that many UKIP voters are erstwhile Conservatives on the rebound."
"Large proportions are negative about David Cameron and George Osborne on the economy, and about Mr Cameron’s handling of gay marriage," he said.
:: ComRes interviewed 2,002 GB adults online between December 12 and 14.