David Cameron last night issued a desperate appeal to voters in the Rochester by-election to vote tactically to keep out Ukip – as a poll showed the anti-EU party with a 12-point lead.

In a highly unusual move, the Prime Minister urged Labour, Liberal Democrat and even Green supporters to lend their votes to the Tory candidate Kelly Tolhurst to prevent a ‘Ukip boost, and all the uncertainty and instability that leads to’.

The poll, commissioned by the former Tory treasurer Lord Ashcroft, put Ukip on 44 per cent, the Conservatives on 32, Labour on 17 and the Lib Dems on only 2 per cent.

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David Cameron, on a visit to Strood Academy in Rochester, urged Labour, Liberal Democrat and even Green supporters to lend their votes to the Tories

A new poll by Lord Ashcroft puts Ukip comfortably ahead of thee Conservatives by 12 points

The finding represents a slight narrowing of Ukip’s lead since the last survey a fortnight ago, when it was 15 points.

But it still suggests a comfortable win for Ukip’s Mark Reckless, the Kent seat’s former Tory MP, despite a massive Conservative push.

However, the poll also produced a glimmer of good news for Mr Cameron by showing the Tories could re-take the seat at the general election.

Thirty-six per cent of Rochester voters said they would probably vote Conservative next year, compared with 35 per cent backing Ukip.

This could curb the enthusiasm of some Eurosceptic Tory MPs, who are weighing up whether to follow Mr Reckless and Clacton MP Douglas Carswell by defecting to Ukip.

Defeat in Rochester and Strood on Thursday of next week would be a serious setback for the Prime Minister.

During another visit to the seat yesterday, he made a direct appeal to non-Tory voters to lend their votes to the Conservatives to keep Ukip out.

The Prime Minister said the seat was now a ‘two-horse race’ between Ukip's Mark Reckless (left) and Tory candidate Kelly Tolhurst (right)

Mr Cameron told the Kent Messenger that the seat was now a ‘two-horse race’. Voters faced a ‘very clear choice’.

He said: ‘You can vote for Ukip and be part of the national campaign and another notch for them in their development and then the great caravan will move on, or you can vote for Kelly, who is a hard-working person, born and raised locally.

‘I would say to people who have previously voted Labour, Liberal, Green or anything, that if you want a strong local candidate and don’t want some Ukip boost and all the uncertainty and instability that leads to, then Kelly is the choice.’

Ukip said it was extraordinary that Mr Cameron was trying to ‘shoehorn Labour voters into his survival strategy’.