The gay former policeman likely to be confirmed today as the Liberal Democrat candidate for London mayor will publish a “tell-all” autobiography three weeks before polling day, The Times can reveal.

The timing of Brian Paddick’s book Line of Fire, which earned him a sixfigure advance, is causing unease among Liberal Democrats, who do not know its contents yet. According to the Simon & Schuster website, the publication date is April 7, which falls at a critical juncture in the campaign for London mayor, which culminates in polling day on May 1.

Mr Paddick has been competing for the candidacy against Chamali Fernando, a barrister from Finchley, and Fiyaz Mughal, chief executive of Enfield Citizens Advice Bureau. The result will be announced this morning and the winner will be battling Ken Livingstone for Labour and Boris Johnson for the Conservatives.

Mr Paddick has already said that the book will be sensational. “In the book there will be nothing hidden at all,” he said in a recent interview. “You will be very surprised by some of the things in it, I can assure you.”

It would reveal “the sort of experiences I have had, both professionally and privately”, he added.

Mr Paddick gained prominence in 2002 when, as Commander for the London Borough of Lambeth, he instructed his officers not to arrest or charge people who were found to be in possession of cannabis. In another episode he was criticised for saying that he found the concept of anarchy attractive. When he left the Metropolitan Police he was the highest-ranking openly gay officer. Andrew Gordon, his publisher at Simon & Schuster, said when the deal was announced: “Brian has left the force and has no restraint on what he can say apart from the libel laws. He will say exactly what he thinks about how the Met is run.” He is understood to be hoping for a newspaper serialisation deal.

Simon & Schuster has made him sign a confidentiality clause in the agreement.

There are fears among some that Mr Paddick will use the 352-page book to settle scores with his former boss, Sir Ian Blair, the beleaguered Commissioner of the Metropolitan Police. Mr Paddick and Sir Ian fell out over the commissioner’s handling of the shooting of Jean Charles de Menezes at Stockwell Underground station. Mr Paddick has alleged that the Home Office intervened to ensure that Sir Ian was not brought down.

In an article this month, he said: “Officers asked me recently how my autobiography is coming along and when I said I thought the commissioner might not like it, they rubbed their hands in anticipation.”

Mr Paddick promised last night to share the contents of the book with his party in advance but said that it would have been premature to do so before he became the official candidate. He said that he had not been informed by his publishers of the publication date and denied that there was any conflict of interest between running for mayor and publishing the book. “Not that I can see,” he said.

A Liberal Democrat source said that the issue of the book would have to be dealt with.

Mr Paddick’s initial failure to call for Sir Ian to resign has perplexed some in the party because Nick Clegg, the Liberal Democrat home affairs spokesman and leadership contender, was the first leading figure to do so.

Earlier this month Mr Paddick said in an article for The Mail on Sunday that he had done a deal with Sir Ian. He said: “I met him privately ten days ago in his office at Scotland Yard. He was sanguine about the healthand-safety prosecution and said he would not resign, whatever the verdict. I promised him I would not publicly call for him to step down and I will be true to my word.”

He then went on, however, to be deeply critical of Sir Ian in the same article, suggesting that he had devalued the office and turned the shooting into a political and police disaster, and saying: “The system has proved unwilling effectively to hold individuals to account.”

The two candidates competing to be Liberal Democrat leader traded claims yesterday over their level of support. Chris Huhne, the environment spokesman, said that he had nearly twice as many supporters as Nick Clegg. Mr Clegg trumped him this afternoon by announcing that he had signed up more than 1,500 members to his opponent’s 1,045.

Ballots will be sent out to more than 64,000 members on November 21 and the next leader will be announced on December 17.

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