TORY MPs in marginal seats where Labour is the challenger – including one in the North-East – are facing disaster, a groundbreaking poll found.

The survey gave Labour a hefty 14-point advantage in the 32 seats which have the smallest Conservative majorities over Mr Miliband's party.

Those 32 include Stockton South, where James Wharton will defend a majority of just 332 over Labour at the 2015 general election.

The Tory high command is so confident that Mr Wharton will win again that his seat is not among the 40 ultra-marginal seats where candidates will receive extra help.

But Lord Ashcroft, the former Tory deputy chairman – who polled 13,000 voters in just 40 tightly-fought seats – found Labour was poised for victory in Con-Lab marginals.

The key factor was the rise of the UK Independence Party (Ukip), which was winning over Tory voters, putting Labour firmly in the lead.

Lord Ashcroft said: “Though this is only a snapshot, the picture is clear. Despite their narrow national poll lead, Labour are further ahead in the marginals where it matters.

“Labour have made progress in the last two years, but if Ukip does as well at the general election as this poll suggests, Ed Miliband could become prime minister with a comfortable majority.”

Labour's 14-point lead in their target seats - 43 per cent to 29 per cent - compares with a national lead of just five points in a poll of the whole country.

However, the poll did suggest the Tories will hold seats, such as Harrogate and Knaresborough, where they are defending narrow majorities from Liberal Democrats.

In Stockton South, Mr Wharton will hope to ward off the Ukip rise through his Bill to guarantee a referendum on EU membership, in the next parliament.

The poll is groundbreaking because 300 people were polled in each target seat, compared with 1,000 across Britain, in a typical survey.