An extensive poll of more than 26,000 voters in 26 marginal seats found a 6.5 per cent swing away from the Tories – a strong enough move to topple 83 Conservative MPs if replicated across the country in next year’s General Election.
The poll suggests the Labour leader would be installed as Prime Minister as one of 340 Labour representatives – enough MPs to form a Government in the 650-seat House of Commons.
Conservative representation would fall from 304 seats to 221, meaning that David Cameron would have to leave Downing Street.
The poll commissioned by the former Conservative Treasurer Lord Ashcroft surveyed 1,000 people in each of the 26 marginal constituencies, where the General Election will be won and lost.
It found that 41 per cent of those surveyed intended to vote Labour compared to 29 per cent who said they would support the Tories.
It also reflected the sensational rise of Ukip which was seen in the English local elections and is expected to be confirmed today when European Election votes are counted.
The poll found Ukip support to be at 18 per cent, dwarfing Lib Dem support which was just eight per cent.
The figures suggest that Miliband could even command a slender majority in the House of Commons if Scotland voted Yes and Labour lost its 41 Scottish MPs.
Were Scotland to go independent, Labour would still manage to return 298 MPs in what remains of the UK – a figure just beyond the 296 MPs required for a majority in a House of Commons shorn of its 59 Scottish seats.
The poll is welcome news for Labour, who ran a lacklustre campaign for the European and English local government elections.
Despite Labour emerging as the largest party in the local government elections, Ukip ate into the gains that Miliband’s party was expected to make.
Labour increased its number of councillors by 338. The Tories lost 231 local Government seats. Meanwhile Ukip gained 161 seats largely at the expense of the Conservatives, although Labour was also hit by the rise of Nigel Farage.
Ukip’s emergence as a serious political force saw senior Tories suggest that Cameron do a deal with Farage in an attempt to prevent Miliband profiting from a split in the right-wing vote.
Miliband faced criticism from Labour members who thought the party did not do enough to counter the Ukip threat.
The Labour leader himself also came under fire for his personal performance. His failure to give an accurate estimate of a weekly grocery bill led to accusations that he was out of touch. He also took part in a toe-curling radio interview that saw him fail to identify one of his council candidates.
Despite Labour’s lead, the poll showed that fewer than three in 10 voters in the marginal seats said they would rather see Miliband as Prime Minister than Cameron.
Last night, a Scottish Labour spokeswoman said: “This poll along with the results of the local elections in England show that in local communities up and down the country Labour’s commitment to tackling inequalities and helping families cope with the cost of living crisis is striking a chord with voters.
“In Scotland we are committed to expanding the powers of devolution as part of a strong United Kingdom and our experience on the doorsteps indicates that majority of Scots are supportive of our plans.”
The poll will add weight to Labour’s narrative that a Miliband-run UK Government is the best way to rid Scotland of Conservative-rule.
Labour has deployed that argument in an attempt to counter the SNP’s claim that independence offers a break from a Cameron’s Government.
The poll was conducted in the 12 most marginal seats held by either Conservative of Labour.
Ashcroft also added two seats – Great Yarmouth and Thanet South – which had slightly bigger Tory majorities and were likely to be on Ukip’s target list.
The poll indicated that the Lib Dems’ vote share had fallen by more than half in the marginal seats since the 2010 General Election.
Those who had voted Liberal Democrat in 2010 were now more likely to say they would vote Labour than for Nick Clegg – although nearly one fifth were undecided.
Thirteen per cent of Labour’s vote in the seats that are key battlegrounds for both Conservative and Labour came from people who said they voted Lib Dem last time round.