Tory poll has bad news for the PM: Labour will win the next election

David Cameron scored dismally in a poll among party activists (PA)

David Cameron's hopes of holding on to power beyond the next election suffered a fresh blow last night, as a new poll of marginal constituencies suggested Ed Miliband was on course to win an 84-seat majority in 2015.

The extensive poll, by senior Tory Lord Ashcroft, showed Labour would gain 109 seats – 93 of them from the Conservatives – if an election were held tomorrow.

The results, presented at a conference in London on how the Tories could win in 2015, came after a poll of party activists showed only 7 per cent felt Mr Cameron would still be in 10 Downing Street after the election.

Party officials last night played down the significance of the findings, claiming that they were due to temporary "mid-term blues", rather than a longer-term dissatisfaction with the performance of the Tories in government.

But the dismal polling will intensify backbench disillusionment with Mr Cameron and his alleged preoccupation with "peripheral" concerns, including gay marriage, instead of mainstream Conservative issues.

Nigel Farage, the leader of Ukip, yesterday suggested his party would only consider an electoral pact with the Tories if they ditched Mr Cameron – whom he described as "a con man" who could not be trusted to deliver his promised referendum on the European Union in 2017.

The Prime Minister, who is also embroiled in a spat with the Office for Budget Responsibility over claims that he misrepresented their views to bolster his argument on the economy, has been under mounting internal pressure since the Tories came third in the Eastleigh by-election earlier this month.

One of his closest allies, Nick Boles, yesterday attempted to counter the growing dissent by warning against swinging to the right after the humiliating defeat. But he conceded that the party had "screwed up" in Eastleigh.

"Where was the hope? It was as if modernisation had never happened," said Mr Boles, who is a planning minister.

"We screwed it up. We didn't even screw up in a new way. We screwed it up in an old way that we have been doing for a decade. It's so frustrating."

However, although Mr Boles said opponents needed "their heads seeing to" if they believed it was wise to attempt to oust Mr Cameron, the latest poll results add weight to the growing concerns that the Tories are heading in the wrong direction.