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As Tory candidate, he faces a gruelling contest against Labour's incumbent, Ken Livingstone.

The question in many people's minds is whether this most colourful of politicians could withstand the scrutiny involved and stay "on message" for the next seven months.

The man who succeeded Michael Heseltine as Tory MP for Henley in 2001 has one of the highest profiles at Westminster.

Mr Livingstone has admitted that Mr Johnson would be a "formidable" opponent and has got his attacks in early by questioning his seriousness.

Apology

For Mr Johnson, famous enough to be known by most people as "Boris", has a history of what his critics call "gaffes".

In October 2004, while editor of The Spectator, he apologised for an unsigned editorial which criticised the city of Liverpool over grief expressed for Ken Bigley - the British contractor who was taken hostage and later killed in Iraq.

The next month he was sacked as Tory culture spokesman for failing to tell party leader Michael Howard the truth about claims he had had an affair.

And Mr Johnson managed to annoy school dinners campaigner Jamie Oliver by saying he would like to "get rid of [him] and tell people to eat what they like".

An article in April describing Portsmouth as "too full of drugs, obesity, underachievement and Labour MPs" was not universally well received.

Nor was the population of Papua New Guinea best pleased to be associated in a newspaper piece "with orgies of cannibalism and chief-killing".

Classical scholar

Despite his scrapes, Mr Johnson, a friend of fellow Old Etonian Tory leader David Cameron, is highly intelligent and has a following which extends beyond normal politics.

A classical scholar, he has appeared on shows such as Have I Got News For You several times, endearing many with his bumbling, fogeyish persona.

But whether this eccentric-but-establishment image would go down well with London's ethnically and culturally diverse voters remains to be seen.

Alexander Boris de Pfeffel Johnson was born in New York in 1964.

He was educated at Eton before studying classics at Oxford where he also became president of the Union.

After leaving university in 1987, he worked briefly as a management consultant before becoming a trainee reporter with the Times newspaper.

His journalistic career suffered an early setback when he was sacked for falsifying a quote.

He then worked for the Wolverhampton Express and Star before joining the Daily Telegraph where he became the paper's European correspondent, working in Brussels.

He spent five years in that job and built a reputation as a good, if occasionally maverick, reporter.

On his return to London he became the newspaper's political commentator and in 1999 was appointed editor of the right-of-centre Spectator magazine, which he left in 2005.

After just six years at Westminster he has become more famous than most Cabinet ministers, without ever soaring to the top ranks of even his own party.

Mr Johnson will face huge scrutiny in the run-up to next May's mayoral election - and journalists are salivating over a ding-dong contest between "Boris and Ken."