Gordon Brown's attempted fightback has failed to win over voters ahead of this week's crucial Crewe and Nantwich by-election, according to a new poll.
Labour is 17 points adrift of the Conservatives, the ComRes survey for The Independent on Sunday shows.
The Tories are on 43 per cent, up three from last month. Labour is unchanged on 26 per cent, while the Lib Dems are down one on 19 per cent.
The £2.7bn package to offset the controversial axing of the 10p tax rate has not put right the problem, most of the public say.
The first poll since the tax change announced by Alistair Darling is yet another damaging blow to the Prime Minister. Some 49 per cent of voters say that it will not make a difference, compared with 39 per cent who think it will.
Nearly six out of 10 – 57 per cent – believe the Labour Party must change leader if it is to have a chance of winning the next election. Just 36 per cent disagree. Some 43 per cent of Labour voters think the party should ditch Mr Brown.
The 17-point Tory lead is the largest in a ComRes poll and the fourth highest overall since 1987, when Margaret Thatcher was at the peak of her power. It would give David Cameron a 128-seat majority in a general election. Labour insiders now say they need a miracle to hold on to the 7,078 majority in Crewe on Thursday.
Last night an ICM poll in the constituency for the News of the World gave the Tories an eight-point lead, putting them on 45 per cent to Labour's 37 per cent.
A national YouGov survey for The Sunday Times puts the Conservatives 20 points ahead on 45 per cent to 25 per cent.
The constituency will be flooded with dozens of ministers and senior Tories and Lib Dems as the most important by-election in Labour's 11 years in government draws near.
The Prime Minister tried to evoke one of the high points of his premiership yesterday with a speech reminiscent of his address to the Labour party conference last September, the last time his party enjoyed a double-digit lead. Mr Brown attempted to remind voters of his personal attributes and the "moral compass" underpinning his political beliefs in a speech to the Church of Scotland.
In a nod to his political troubles the Prime Minister, the son of a Church of Scotland minister, said the "challenges of recent months" had taught him "what really matters".
The poll offers a crumb of comfort for Mr Brown, as voters are split on whether Mr Cameron is the right person to lead the country.
Some 46 per cent think the Tory leader has what it takes to be a good Prime Minister, only four points ahead of those who disagree. Voters are similarly divided over whether Cherie Blair was right to publish her memoirs now at a time of embarrassment for the Prime minister: 43 per cent were in agreement and 45 per cent against.
Frank Field, the leader of the 10p tax rebels, tells The Independent on Sunday today that he remains "genuinely puzzled" that Mr Brown had axed the 10p band of income tax in his final Budget as Chancellor.
He reveals that the Prime Minister thanked him in a telephone call after Mr Field apologised for describing Mr Brown as "so unhappy in himself".
In his speech to the Church of Scotland Synod in Edinburgh, the Prime Minister said: "For me, a life is best measured not by what office or title you hold but by what difference you can make by seeking to do what you judge the right thing, however difficult, and by the causes to which you dedicate your efforts."
Mr Brown highlighted the capacity of individuals to force change as he criticised the uncaring and "unnatural" military dictatorship in Burma.
The speech contrasted with the "Sermon on the Mound" delivered by Mrs Thatcher 20 years ago. The then prime minister provoked criticism with an address on the issue of personal responsibility to members of the Church of Scotland. But Mr Brown had a warm reception and was loudly applauded.
Meanwhile, it was reported last night that Mr Brown has moved to slap down two Cabinet ministers in a bitter Government row over plans to charge householders a controversial "pay-as you-throw" rubbish tax.
The Prime Minister was furious after Hazel Blears, Secretary of State for the Communities, and Hilary Benn, Secretary of State for the Environment, last week described pilot schemes as a "sensible approach".
Mr Brown has now made it clear that a nationwide "bin tax" of up to £100 a year will not be levied. "It will be brought in over Gordon's dead body," a Downing Street source said last night.