Gordon Brown's standing with the public has been damaged by the plan by one of his closest aides to smear senior Tories, according to The Independent's "poll of polls".
The latest slide in support for Labour started immediately after the revelation that Damian McBride discussed proposals for a website, Red Rag. "Much of the enhanced personal popularity Gordon Brown gained last autumn has now been lost," said John Curtice, professor of politics at Strathclyde University, who compiled the "poll of polls".
At the end of a torrid week for Mr Brown, his leadership came under further fire from anxious Labour backbenchers. Charles Clarke, the former Home Secretary, said: "There have been been things that have been done recently that have made me feel ashamed to be a Labour MP ... There's a lot [of MPs] around saying that Gordon has to improve his performance."
Prof Curtice said the "smeargate" affair had an instant impact in the polls, with Labour's ratings slipping by an average of three points to 28 per cent in the surveys taken after it, before falling a further point following the Budget.
Prof Curtice said political rather than economic events moved the polls. "On this occasion it was Mr Brown's failings that were on display – in particular the people from whom he takes advice and schemes on his behalf and his difficulty in acknowledging mistakes," he said.
"These are very similar failings to those on display during the  election that wasn't and the 10p tax row. Even if the economy were to turn around by next year, as forecast by the Chancellor, it will profit Labour little if such political mistakes keep being repeated."
The weighted average of the polls taken in April puts the Tories on 43 per cent (up two points since March), Labour on 28 per cent (down two points), the Liberal Democrats on 19 per cent (up one point) and other parties 10 per cent (down one point). If repeated at a general election, these figures would give David Cameron an overall majority of 104.
"Labour's position has become almost as dire as it was last summer, when it was the most unpopular Labour Government in history," said Prof Curtice. He found no consistent evidence that Labour's economic credentials were boosted by last month's G20 summit in London. But he added: "Despite the Government's current woes, the battle for economic competence is still to be finally won and lost."
Yesterday Cabinet ministers dismissed speculation that Mr Brown could face another leadership crisis if Labour does badly in next month's European and local elections. They accused Labour backbenchers of behaving like "headless chickens".
But there were also squabbles among ministers over who should take the blame for the Government's humbling Commons defeat over residence rights for Gurkha veterans on Wednesday. Allies of Jacqui Smith, the Home Secretary, denied she miscalculated the strength of feeling among MPs on the issue.
Labour MPs continued to express doubts about the Prime Minister. John McFall, chairman of the Commons Treasury Select Committee, called for more "long-term vision" and said Mr Brown realised he "could have done better" in recent days, and Bob Marshall-Andrews, MP for Medway, described Thursday's votes on MPs' expenses as a debacle.
The Government came under more pressure when the Liberal Democrats uncovered figures showing there are more than 10 jobseekers for every vacancy in more than half the parliamentary constituencies in the UK – double the number a month earlier.
Steve Webb, the Liberal Democrat spokesman on work and pensions, said Mr Brown was "divorced from the reality facing jobseekers".