The Conservatives have doubled their lead over Labour from seven to 14 points following the row over Gordon Brown's decision to abolish the 10p lower rate of income tax.
The latest ComRes survey for The Independent, taken between Friday and Sunday, puts the Tories on 40 per cent (up two points since last month), Labour on 26 per cent (down five points), the Liberal Democrats on 20 per cent (up three points) and other parties on 14 per cent (unchanged).
The 14-point lead is the biggest enjoyed by any party since ComRes began polling for this newspaper in September 2006. The previous highest was a 13-point Tory advantage in November.
It suggests the controversy over the 10p tax rate has damaged Labour's standing among its traditional working-class supporters ahead of Thursday's local elections in England and Wales.
When pressed yesterday on whether he regretted the fact that some poorer voters would lose out on the 10p tax, Mr Brown said: "Of course, because it's unfortunate when things go wrong for people and we've tried to sort that out immediately over the last few days.
"But I do stress, as a result of the Budget, far more people are better off, people who are on low incomes are better off, people who are in poverty as families or as pensioners, many have been taken out of tax altogether as a result of the decisions that we have made."
David Cameron attempted to capitalise yesterday on the Government's continuing discomfort on the issue, publishing a dossier he said highlighted Labour's failure to tackle poverty in Britain. The Tory leader said child poverty had risen by 100,000 in the past year and insisted that Mr Brown's ambition to cut poverty had failed.
Mr Cameron said: "No one doubts his sincere desire to remove poverty from our land. Of course, he wants to. But his delivery has been desperately disappointing. His rhetoric doesn't reflect the reality. His great passion has ended in great failure."
According to the ComRes poll, the Tories are now ahead of Labour among all social classes, including the bottom DE group, among whom they have a lead of 36 per cent to 34 per cent. Last month, before the row erupted, Labour was ahead by 37 per cent to 30 per cent among this group.
Among C2 skilled manual workers, who propelled Margaret Thatcher to power, the Tories are on 40 per cent and Labour 23 per cent. Last month, the Tories were only one point ahead (by 33 per cent to 32 per cent). If the results of the latest poll were repeated at a general election, Mr Cameron would win an overall majority of 62.
However, the survey shows that the Liberal Democrats have profited more from Labour's troubles than the Tories. According to ComRes, the third party has moved beyond 17 per cent for the first time since Nick Clegg became its leader in December.
Labour's supporters are also the least loyal to their party. While 94 per cent of people who identify themselves as Tories and 93 per cent of Liberal Democrats intend to vote for those parties, only 78 per cent of Labour identifiers say they will support the party.
Only 53 per cent of people say they are certain to vote at a general election, suggesting that many have switched off from politics.
ComRes telephoned 1,005 British adults on 25-27 April. Data were weighted by past vote recall. ComRes is a member of the British Polling Council and abides by its rules. Full tables at comres.co.uk.