David Cameron has been warned by his general election guru that the Tories risk losing power in 2020 unless they cut immigration, according to a new book.

Lynton Crosby, the Australian political consultant widely credited with delivering the surprise victory, questioned a selected group of 2,000 people in the weeks after polling day to find out why they voted for Mr Cameron.

He found voters believed the party had been regarded as "best able to manage the economy, spending and debt" and offering "strong and stable government" in contrast to a potential Labour-SNP coalition.

Increasing NHS spending, delivering an EU referendum and clearing the deficit were also listed as priorities.

But Mr Crosby found that the policy area the group wanted Mr Cameron to address above all others was migration.

The strategist presented results of the confidential poll to the new Cabinet during the summer.

He told ministers that keeping the promise to "control and reduce immigration" would be crucial to winning the 2020 election.

The warning emerged in Why The Tories Won, written by Telegraph journalist Tim Ross and published by Biteback.

Other details revealed include the frustration of Ed Miliband's team at his "grumpiness" early in the morning, which meant morning strategy meetings had to be scheduled some two hours later than the Tories held theirs.

The notorious photographs of the Labour leader eating a bacon sandwich were blamed on the fact that Mr Miliband was not a "morning person" and rejected advice to wait to eat his breakfast.

There is also a suggestion that Mr Miliband was the driving force behind the interview with Russell Brand. The comedian initially offered a simple endorsement but Labour wanted to show that Mr Miliband had worked to win him over.