The Conservative leader spent 30 minutes addressing sixth- formers at South Hunsley School, Melton, on the topic of civil liberties alongside David Davis, while various of the other 25 candidates waited outside trying to attract media attention.

Mr Cameron said he was unlikely to visit the Haltemprice and Howden constituency again before the election. Nor would he promise Mr Davis another job should he be re-elected.

Mr Cameron also admitted there had been a "difference of opinion" with Mr Davis over whether he should quit his seat.

The Tory leader said: "If everyone quit and held a by-election on issues they felt strongly about then I wouldn't have much of a shadow cabinet left. But David felt very strongly and I'm here to support him."

Asked over why the Tories did not back the Government's attempts to increase detention-without-charge to 42 days, Mr Cameron said that he would prefer to be right than popular.

"My strong suspicion was that our Prime Minister was planning to change the number of days' detention for political reasons."