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The Australian strategist behind Boris Johnson's mayoral campaigns is to work for the Conservatives, Tory chairman Grant Shapps has said.

Lynton Crosby will start working part-time as a campaign consultant in January and step up his involvement before the general election.

Mr Crosby first worked for the party on the unsuccessful 2005 election campaign when Michael Howard was leader.

Mr Shapps said he was a "serious campaigner" who would bring "focus".

91�ȱ� political correspondent Chris Mason said the Australian was set to become a "big figure" in the Conservatives' strategy for the next election.

Mr Crosby is known for being skilled at pushing "subliminal messages" in areas such as immigration and violent crime, our correspondent said.

But his appointment could prove controversial. Last month former deputy party chairman Lord Ashcroft said it would be a mistake to hire Mr Crosby, who could be a "recipe for... conflict and confusion".

Some in the party feel he is too right-wing to appeal to more moderate British voters, while others think the prime minister has enough advisers and the appointment is unnecessary, our correspondent said.

Mr Shapps told the 91�ȱ�'s Sunday Politics: "This is a serious campaigner. Lynton brings the kind of focus that's required to manage campaigns and we have a big job to do to explain what's going on.

"This country is in a global race. We need to be able to get out there, show that we can secure a brilliant future for Britain."

Mr Crosby previously worked on a string of successful campaigns with former Australian prime minister John Howard.

He first worked for the Conservatives on Michael Howard's unsuccessful election campaign of 2005, which focused on crime and immigration.

But he went on to work with Mr Johnson on his more successful London mayoral campaigns.

His appointment has been welcomed by Mr Johnson, who described him as "the best campaign manager I've ever seen".

Earlier this month he called for Mr Cameron to "break the piggy bank" to get Mr Crosby to work for him.

He suggested then that the Australian would be portrayed as a right-wing "attack dog" by the media but insisted he was the "soul of sweetness".

His appointment comes after the Conservatives lost the parliamentary seat in the Corby by-election, in a vote which saw a 12.7% swing to the Labour Party.

It was the first time the Tories had lost a mid-term by-election seat to Labour since 1997.

'Alpha male'

The elections for police and crime commissioners also suffered the lowest turnout in peacetime Britain, with just under 15% of people turning out to vote on Thursday.

In addition, an opinion poll of more than 2,000 people by ComRes has given Labour a 12-point lead over the Tories - the widest margin of advantage in a ComRes survey for more than seven years.

The poll was conducted for the Independent on Sunday and Sunday Mirror.

Tim Montgomerie, editor of political website Conservative91�ȱ�, wrote that Mr Crosby had "a good record of success and is perhaps the alpha male that Team Cameron has been missing".

Conservative commentator Iain Dale told 91�ȱ� News the appointment was a "fantastic move" for the party and Mr Crosby had a "great track record".

He said the prime minister and Mr Crosby had not always "seen eye to eye" and sparks could fly between the two men, but he was "exactly the right man for the job".