Boris signs up Tory election guru to boost bid for Mayor

Mr Crosby, who starts as campaign strategy chief early next month, has decades of experience of successfully fighting elections in Australia.

But the strategist was also hired by the Tories at the last general election and oversaw a dismal defeat for Michael Howard. He will be paid an undisclosed sum - which could reach as high as six figures - from London campaign coffers for his guidance.

Mr Lynton is regarded as a formidable professional who insiders believe will not shy away from taking on Ken Livingstone.

During the last election he was criticised for focusing on "dog whistle" issues - sending coded messages to specific groups on subjects such as immigration - but has played down the claim. Mr Crosby, who has been described as a "master of the dark political arts", is known for his aggressive style and willingness to "go negative". He is certain to toughen up the public image of the mild-mannered and jovial Mr Johnson.

His appointment will give Mr Johnson's campaign, which had been criticised for its lacklustre start and dearth of big names, a much-needed boost.

The Tory candidate has also hired heavyweight strategists James McGrath, adviser to shadow chancellor George Osborne, and Toby Vincent, a senior London Tory.

Mr Osborne, also the party's campaign chief, is understood to welcome the appointments, although "Team Boris" insists hiring Mr Crosby and his fellow strategists was their idea.

Dan Ritterband, who some Tories felt lacked political experience and clout, remains as campaign director and will maintain overall control of strategy.

Mr Johnson said: "Londoners deserve better than Mayor Livingstone and I am determined to mount the most vigorous campaign possible.

"We are contending against an army of taxpayer-funded spin doctors and now is absolutely the right moment to go up a gear."

One campaign insider, who believes Mr Crosby's appointment will silence internal critics, said: "We're delighted Lynton is coming on board. He'll be helping us prepare strategies for London. We recognise that Ken has a big machine and we need big-hitters to fight back."

The 51-year-old strategy guru has already been fully briefed on Mr Johnson's policy priorities and has read all his speeches and articles on the campaign to date.

He will travel between London and Australia - where he has other work commitments - but will be back in the capital full-time in the crucial weeks running up to the election in May.