Profile: Boris Johnson
Boris Johnson's decision to campaign for Britain to leave the EU is being regarded as a huge boost for the Out campaign.
The 51-year-old London mayor has spent more than a decade combining the gravitas of his various roles with an unkempt, humorous persona rarely seen in modern public life.
Prime Minister David Cameron expressed his delight when Mr Johnson announced his decision to run for parliament again in 2015, saying: "I want my star players on the pitch."
But now the MP for Uxbridge and South Ruislip has said he will campaign against the PM in the EU referendum, relations between the two may become more tense.
The big question is whether the blond-haired old Etonian wants simply to play for the team or to captain it.
He has arguably the highest profile of any Conservative except Mr Cameron and is thought by many to harbour an ambition to be prime minister.
When he beat Labour's Ken Livingstone to become London mayor in 2008 it was the Tories' first high-profile election success since before Tony Blair's triumphant entry into Downing Street in 1997. He defeated Mr Livingstone again in 2012, giving him even more of a winner's aura.
The hoopla surrounding Mr Johnson broke out again when he ended months of speculation by confirming he would try to return to the Commons as MP for Uxbridge and South Ruislip.
He swept into the safe Conservative London seat in the 2015 general election, taking a spot at the cabinet table as minister without portfolio.
Given the opportunity to enter Downing Street on a fortnightly basis, cycle helmet in hand, was seen as recognition for his unique rapport with voters.
But crucially he is not bound by collective cabinet responsibility, giving him the freedom to rebel against controversial decisions without having to resign.
Although lauded by Conservative activists for his witty speeches, Euroscepticism and lack of PR polish, Mr Johnson's political life has not been blemish-free.
In 2004 he had to make a visit to Liverpool to apologise for an article in the Spectator magazine, which he then edited. It had criticised the people of the city for their reaction to the death of Ken Bigley, the British contractor taken hostage and killed in Iraq.
The following month he was sacked by Conservative leader Michael Howard for failing to tell the party the truth about claims he had an affair.
He created media hysteria at the 2006 Conservative Party conference when he attacked healthy eating advice advocated by the chef Jamie Oliver. He said he would like to "get rid of [him] and tell people to eat what they like".
Later he provoked anger by describing Portsmouth as "too full of drugs, obesity, underachievement and Labour MPs" and associating Papua New Guinea "with orgies of cannibalism and chief-killing".
The London mayor's life has been as exotic as his use of language.
Alexander Boris de Pfeffel Johnson was born in New York and he held US citizenship until 2006. Descended from Turkish, French and German stock he describes himself as a "one-man melting pot". His great-grandfather, Ali Kemal, briefly served as an interior minister in the Ottoman Empire.
The son of a diplomat and Conservative Member of the European Parliament, he was educated at Eton College, where he was senior to Mr Cameron. There is said to be a sense of resentment on Mr Johnson's part at being overtaken by his younger rival.
At Oxford University, Mr Johnson, who flirted briefly with the centre-left Social Democratic Party, was President of the Union. After only a week as a trainee management consultant he moved to journalism. Fired from the Times for making up a quote, he then worked for the Wolverhampton Express and Star before joining the Daily Telegraph, working as its Brussels correspondent and assistant editor.
In his seven years as MP for the ultra-safe Conservative seat of Henley from 2001 he did not make the shadow cabinet, becoming a party vice-chairman and holding roles as a junior spokesman on education and culture.
His decision to run for London mayor in 2008 added excitement to the contest. However, during the campaign he made a conscious effort to avoid giving reason for accusations of eccentricity. Most Londoners seemed to approve, as he beat Mr Livingstone by 54% to 47%.
Days after his victory he banned alcohol consumption on public transport. A keen cyclist himself, he launched a central London bicycle hire scheme, known colloquially as "Boris bikes", and ditched the "bendy bus" in favour of a new generation of double-deckers.
Although suffering a series of high-profile resignations from City Hall, it was Sir Ian Blair's departure as the Metropolitan Police Commissioner which attracted most controversy. Sir Ian said he was stepping down because of a lack of support from Mr Johnson.
He was in charge when riots hit the capital in the summer of 2011. Initially criticised for taking too long to return from a holiday, he was later praised for his handling of events, including making tours of the affected areas.
Mr Johnson also represented the city when it received its greatest publicity in decades, during the 2012 Olympics. His speeches were well received but he was a little embarrassed when, at one promotional event, he became caught on a zipwire, to the delight of photographers.
In 2011 he told the BBC's Newsnight he did not expect to do "another big job in politics" after leaving the mayoralty. This was not universally accepted at face value.
Mr Johnson has arguably fuelled speculation about his ambitions by continuing to make speeches touching on issues beyond his remit in London, in particular on Europe.
His latest decision means that speculation is guaranteed to continue.