David Cameron has launched a fightback against the Lib Dem poll surge which threatens to derail the chances of a Tory victory on May 6.

Many party activists, including those he addressed in the target Labour seat of Gloucester this morning, are seriously concerned at the potential impact of the third party's rise.

Nick Clegg's strong performance on the first televised debate helped push the Lib Dems into second place in the latest poll - leapfrogging Labour to sit just three points behind the Tories.

The YouGov daily survey for The Sun suggested the contest had become a real three-horse race, with the Lib Dems on 30 per cent to Labour's 28 per cent and the Conservatives in the lead on 33 per cent.

Thanks to the complexities of the Westminster voting system, if that was repeated across the country the Tories could end up fewer seats than Labour.

Mr Cameron told supporters they had "a great fight on our hands". Only a "decisive" Tory win at the ballot box would "get things done" to solve the country's problems, he said.