Ukip leader Nigel Farage has delivered a withering response to a plea by David Cameron for people to vote tactically in the Rochester and Strood by-election.

Mr Farage said the Prime Minister was clutching at straws and his appeal to supporters of other parties to vote Conservative to deprive Mark Reckless of victory was desperate.

In an interview with the KM Group, Mr Cameron said he wanted people to consider backing Kelly Tolhurst to deny Ukip victory.

He said: "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.

"By-elections are different; there is a chance for people to vote in way they haven't done before."

But Mr Farage said: "For a Prime Minister to beg voters who are by no stretch supporters of his, to gang up against the underdog is comical, for him to cite 'uncertainty and instability' as the reason is risible.

"The only uncertainty and instability that he fears is in the Tory ranks and whether he can continue to carry his own back-benchers."

"The Tory campaign on the Medway is rudderless, becalmed and now it seems without either a captain or a crew."

Ukip candidate Mark Reckless said: "Every time David Cameron, his ministers, and his MPs come to this constituency they show us that, for them, this election is not about the people of Rochester and Strood, but about their own political careers, and whether they can survive a shift to Ukip here."

Mr Cameron's appeal for tactical voting came as a new poll put the Ukip ahead of the Conservatives by 12 points.

The poll by Lord Ashcroft put Ukip on 44%, the Conservatives on 32% and Labour on 17%. The Greens are on 4% with the Lib Dems trailing on 2%.