Boris Johnson used a threat of troublemaking at the 2011 Tory conference to lever a promise of £93 million for policing in London from Chancellor George Osborne, former party donor Lord Ashcroft has claimed.
Lord Ashcroft said David Cameron and Mr Osborne were so concerned about the ability of the London mayor to make mischief for the leadership, the Chancellor quickly conceded to his demands.
The claim appears in the latest instalment of the billionaire peer's unauthorised biography of Mr Cameron - Call Me Dave - co-written with the journalist Isabel Oakeshott, which is being serialised in the Daily Mail.
The incident was said to have taken place in the days leading up to the 2011 Conservative Party conference, which came weeks after summer riots raged across a number of English cities.
Mr Osborne was said to have been worried that his big set-piece speech would be overshadowed by Mr Johnson's regular column for the Daily Telegraph which was due to appear the same day.
According to the book, Mr Osborne telephoned the mayor to appeal to him not to cause trouble.
"We just want a quiet conference. Nothing unexpected," the Chancellor reportedly told him.
Mr Johnson reportedly replied: "Hmm, funny you should say that. I'm just about to write my column for the Telegraph and I'm staring at a blank page."
When the Chancellor retorted: "Very funny," Mr Johnson was said to have shot back: "No, seriously. What price no mischief?"
Mr Osborne was said to have responded: "What do you want?" to which Mr Johnson replied: "£90 million extra for policing in London."
The result was said to have been a £93 million policing package for the capital, leading Mr Johnson to reportedly joke to aides: "That was the best-paid column ever."
Lord Ashcroft's book has already caused embarrassment for Mr Cameron with allegations that he took part in a bizarre student initiation ceremony involving a dead pig. Sources close to the Prime Minister have insisted they did not recognise the claims.
The peer - who gave the Conservatives £8 million while they were in opposition - has angered many Tories after he disclosed that he decided to write the book when Mr Cameron refused to give him a significant ministerial job following the 2010 general election.