Audience member accused of being a plant for Davis

Mr Cameron's supporters have accused him of being a Davis "plant", whose contribution soured the mood of the programme which had been good natured until Mr Aitken, seated on the front row, threw in a short but pointed contribution.

He initially praised Mr Cameron's party conference performance but then went on to add that, stripped of his slick props, it was clear he "couldn't run a bath".

Mr Cameron looked stunned by the bluntness of the question by the fellow Old Etonian, but his supporters were furious. Last night a Cameron aide said: "He was clearly planted there. It was a really offensive remark and jarred with the programme.''

Mr Aitken, 23, a leading light in Conservative Future, the campaign organisation for Tory students, was said by Cameron supporters to be a Davis supporter and involved in the campaign. Mr Davis was a guest two years ago at Jonathan Aitken's wedding to Elizabeth Harris.

"The programme issued tickets to both sides and we had no trouble filling our quota," said a Cameron campaign member. "The Davis camp seemed unable to find enough ordinary supporters so gave a few to their campaign team activists, including Will Aitken.''

The Davis camp last night said there had been no foul play. "People want to see how the candidates do under pressure with difficult questions. They can't complain if they get something hostile," said a Davis supporter. "Half of the tickets were divided between Davis and Cameron supporters so it should not come as a big surprise that our supporters were in the audience."