By Andrew Osborn
LONDON (Reuters) - High-profile London mayor Boris Johnson's hopes of becoming Britain's next prime minister and leader of the Conservative Party have suffered a setback after a faltering TV interview, allies and detractors said on Monday.
Known for his blunt but colourful language and unruly mop of blonde hair, Johnson, 48, is widely tipped as a front-runner to succeed Prime Minister David Cameron at the helm of the Conservatives if Cameron loses the next general election in 2015 or is ousted as party leader before then.
But Johnson, who is famously self-confident, lost his composure in a BBC TV interview on Sunday when faced with a series of difficult questions about his past. Local media described the embarrassing exchange as a "car crash" and as his worst ever interview.
Instead of displaying his trademark self-assurance, Johnson struggled to respond, stammered, and asked the presenter more than once whether they could talk about something else.
"You're a nasty piece of work, aren't you?" the presenter told Johnson, after alleging that he had once made up a quote when working as a journalist, lied about his personal life, and been complicit in plans to physically assault a journalist.
"Interpretations you're putting on those things aren't wholly fair," a visibly shell-shocked Johnson retorted.
Voters "don't care about phone conversations with my friends 20 years ago, they don't care about some ludicrous so-called made-up quote," he said.
He said he disputed the allegations, but appeared - uncharacteristically - not to be able to find the words to defend himself.
"This was his reckoning, and he looked winded, lumbering like a past-his-prime former heavyweight champion," wrote Liberal Democrat blogger Stephen Tall, echoing a popular view that Johnson had finally come unstuck.
Traditional allies agreed the interview had damaged Johnson's image.
"There's no doubting that Brand Boris is looking a little tarnished," wrote Benedict Brogan, the deputy editor of the right-leaning Daily Telegraph newspaper.
"Boris has so far traded on a collective willingness by the media to overlook his record in order to relish his personality. The danger must be that it will now be open season."
Johnson, whose popularity soared alongside last year's London 2012 Olympic Games, admits he'd like to be prime minister in a documentary about him due to be broadcast later on Monday, saying he'd like to "have a crack" at the job.
Cameron, whose leadership is under pressure ahead of a general election due in 2015, has often laughed off suggestions that Johnson could replace him, once describing the mayor as a "blond-haired mop sounding off from time to time".
He defended him on Monday. "Never attempt to limit Boris's ability to get out of a tight spot," Cameron told reporters.
Tim Montgomerie, editor of the influential ConservativeHome website, said Johnson needed to sharpen up his act.
"I don't know if Boris has become soft but it's vital that he disappears into a locked room with his advisers for a few hours and engages in some US-style debate prep," he wrote.
"He should only re-emerge from that room when he and his advisers are confident that today's car crash won't be repeated."
(Editing by Stephen Powell)