Support for Labour has slumped dramatically in the aftermath of the email smears affair, allowing the Tories to extend their lead to 17 points, according to a new poll.
The survey showed Labour on 26% - down five from three weeks ago and just five points clear of the Liberal Democrats on 21%.
While the Tories were down one on 43%, their advantage over Labour would be enough to give David Cameron a 120-seat Commons majority in a general election.
The findings come amid signs that the row involving resigned Downing Street aide Damian McBride has damaged trust in the Government.
Prime Minister Gordon Brown has been under pressure since it emerged that Mr McBride, formerly one of his key aides, had planned to spread unfounded rumours about senior Tories.
But the poll, conducted by Marketing Sciences Ltd for the Sunday Telegraph, found that three-quarters of voters would not change how they would vote if Mr Brown was no longer leader.
Only 11% said they would be more likely to vote Labour, while 13% said they would be less likely.
A separate poll found that 37% of voters trusted the Government less as a result of "smeargate".
The Prime Minister has insisted that nobody else at Number 10 had knowledge of Mr McBride's emails containing the smears until they were exposed last week.
But Ipsos Mori survey showed that 39% of voters felt Mr Brown's leadership style was responsible for the episode, while 35% did not. Some 54% said such issues were likely to influence the way that would vote, compared to 42% who said they would not.