London's mayor to become prime minister? Not yet
* Boris Johnson feted as a future leader of Conservatives
* Mayor Boris faces complex hurdles to the top job
* UK lawmakers say talk of an early challenge is premature
* Poll suggests Boris could help Conservatives win election
LONDON, Sept 14 (Reuters) - London Mayor Boris Johnson eclipsed British Prime Minister David Cameron as Olympic host this summer, but the eccentric Conservative will have to bide his time before mounting any serious challenge for the top job.
Johnson followed up his Olympic antics -- dangling from a zip line waving the national flag and speeches extolling British success -- with criticism of Cameron's handling of the economy, prompting speculation he was already plotting to replace him.
An opinion poll this week suggested the mayor could revive the flagging popularity of the centre-right Conservatives, who have ruled in uneasy coalition with the left-leaning Liberal Democrats since 2010.
The Conservatives could make up a 13-point lead now held by their Labour opponents if Johnson took over, the YouGov poll for the Sun newspaper showed, reviving talk he may seek, with other dissenters, to oust Cameron before the 2015 parliamentary poll.
Many in the party play that down.
"The whole thing is delightful Westminster gossip, but no more than that," Conservative lawmaker and member of the party's influential 1922 Committee Bernard Jenkin said.
Johnson, a formidable campaigner with a gift for persuading voters of all hues to like him, has no parliamentary seat, so to take over before 2015 he would have to step down as mayor and seize his chance in a bye-election if a seat became vacant.
"In the future, anything is possible but there is not going to be regicide on David Cameron," Jenkin said. "The idea that somehow Boris is going to get into parliament, destabilise the leadership, is just absolutely for the birds."
Johnson, 48, has dismissed as "complete cloud cuckoo land" speculation that he might seek election to parliament during his mayoral term, which finishes in 2016, and said, with his trademark informality, that he would be "clapped out" by then.
But Cameron has come under fire from his party for failing to take a tough enough line on the European Union and steering Britain back into recession while missing a string of his own austerity targets.
The prime minister and finance minister George Osborne have staked their political careers and the party's re-election chances on a return to economic growth before 2015 - both faced the humiliation of being jeered at the Paralympic Games.
By contrast, Johnson's star has kept on rising.
Although a product of the same elite Eton College private school and Oxford University as Cameron, Alexander Boris de Pfeffel Johnson has little obvious in common with him.
Where the prime minister is sharply dressed and slick, on message and politically pragmatic, Johnson twins his scatty manner and wild puff of blond hair with acrobatic use of language and comic turns that give him wide appeal.
Appearances on popular television shows have cemented his position as one of the few celebrities in British politics and the Olympics gave him a global stage.
His chauvinistic speeches -- he said Britain had won enough medals to bail out Spain and Greece -- appealed to voters at home, who view him as a blast of fresh air, even if few could tell you exactly what his politics are.
But any leadership fight could further damage the reputation of the party among voters struggling with spending cuts and recession and grandees say as many people would oppose someone like Johnson as would rally round him.
It would also cast its coalition with the Liberal Democrats into the unknown.
"He's a fantastic asset to the party," said Conservative MP Nadhim Zahawi. "My advice to him would be stay focused on the job (as mayor), deliver for London and the party."
Once in parliament, Johnson would either have to wait for Cameron to resign or engineer a "no confidence" vote by the party, which would require the backing of 15 percent of Conservative lawmakers just to get off the ground.
The party may have internal problems but it was in utter disarray the last time an incumbent Conservative prime minister was ousted from within in 1990.
"Margaret Thatcher's circumstances were so exceptional," Jenkin said, pointing out that she had been in power for 11 years and had angered may in the party by her refusal to agree a timetable for Britain to join the European single currency.
Cameron's situation was different, he said. "The ingredients to a removal of a leader are just not there."
Many Conservatives like Johnson because he speaks his mind freely and goes for broke. Cameron, as leader, has to keep a potentially divisive party together in tough times, a role Johnson may not find so easy.
"It's one thing to make great speeches at the Olympics, to make a virtue of bumbling through them and making the odd gaffe. But when you are in parliament you cannot afford to make those kinds of mistakes - you have to follow a brief," said Tim Bale, author of "The Conservative Party: From Thatcher to Cameron" and a professor at Queen Mary, part of the University of London.
"If he can do that, all good, but that would also lead to Boris not being Boris, and he could lose his appeal."
A journalist before spending seven years as a member of parliament, Johnson has thrived as mayor, free from the shackles of party discipline to pursue eye-catching policies.
His "Boris bikes" bicycle hire scheme proved a winner, and he won support among Londoners across the political spectrum for his opposition to the expansion of Heathrow airport.
While his free market views appeal to core Conservatives, his more liberal attitude to immigration - crucial to his popularity as London mayor - might not, and to come out on top in any leadership fight, Johnson would need to bridge divides.
"He would need to build coalitions within the party," Bale said. "Not only the clown, but also the statesman."