There was more worrying news for David Cameron as two more polls suggested the country is on course for a hung parliament.
Research by YouGov for The Sunday Times found the Tories' lead narrowed from five points to four over the past week.
They were down one point on 37%, while Labour and the Lib Dems were unchanged on 33% and 17% respectively.
If repeated at a general election with a uniform swing across the country, the results would leave Labour as the largest party with 302 seats, against 277 for the Tories, according to the newspaper. However, no party would have an overall majority.
Meanwhile, ICM research for The Sunday Telegraph found the Tories' advantage dipped by two points over the past month - to 38%, compared with 31% for Labour. The Lib Dems were up one on 21%.
The gap is the narrowest in an ICM poll for two years, and suggests Mr Cameron would be in control of the largest party, but 30 seats short of an overall majority.
According to the research, Mr Cameron is more trusted than Gordon Brown to deliver on the NHS (by 2%), schools (by 7%) and the economy (by 3%).
However, in January his leads were 8%, 12% and 7% respectively.
More than half (56%) of those quizzed wanted one party to win a clear majority.
But 34% thought Britain's interests would "best be served" if the election resulted in a hung parliament - despite warnings that such an outcome could panic the financial markets.
