From friends to foes - Boris Johnson goes head to head with Cameron on EU vote

Sterling tumbled on Monday, hitting a two-year low against the yen, as concern grew that Britain would quit the European Union after influential London Mayor Boris Johnson threw his weight behind the exit campaign. Johnson, according to media reports, is making his latest move towards David Cameron's position as prime minister
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Prime Minister David Cameron (L) walks with Mayor of London and Parliamentary candidate Boris Johnson at the Conservative party conference on September 29, 2014 in Birmingham, England. The second day of conference is set to be dominated by the economy. (Getty Images)
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Prime Minister David Cameron (R) stands with London Mayor Boris Johnson as the Olympic cauldron is lit for the Paralympic Games in Trafalgar Square on August 24, 2012 in London, England. The London 2012 Paralympic Games open on August 29, 2012 for 12 days. (Getty Images)
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British Prime Minister David Cameron (L) and London Mayor Boris Johnson leave St Paul's Cathedral in central London on July 7, 2015 after attending a memorial service in memory of the 52 victims of the 7/7 London attacks. Britain today marked 10 years since the London bombings with a minute's silence for the 52 victims, less than a fortnight after an attack in Tunisia highlighted the ongoing Islamist threat. (AFP/Getty Images)
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David Cameron and Boris Johnson shake hands at the end of their tennis match during the International Paralympic Day at Trafalgar Square on September 8, 2011 in London, England. The day marks the start of Paralympic tickets going on sale on September 9, 2011. (Getty Images)
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British Prime Minister Prime Minister David Cameron (R) and Mayor of London Boris Johnson are pictured as they travel by train to Newark in central England, on May 21, 2014, to campaign in the town ahead of the Newark by-election which is due to be held on June 5. Voters also go to the polls in Britain Thursday May 22, 2014, for local council elections in England and Northern Ireland and also for European elections in England, Wales, Scotland and Northern Ireland. (AFP/Getty Images)
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The Mayor of London Boris Johnson (L) and British Prime Minister David Cameron speak as they travel on an underground train to Westminster after visiting Harrow Skills Centre and meeting local shoppers in northwest London, on May 12, 2014, during campaigning ahead of European Parliament and local elections on May 22. (AFP/Getty Images)
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Prime Minister David Cameron and Mayor of London Boris Johnson cheer during the London 2012 Victory Parade for Team GB and Paralympic GB athletes on September 10, 2012 in London, England. (Getty Images)
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Britain's Prime Minister and leader of the Conservative party, David Cameron, (R) and Mayor of London and Conservative party candidate for the Uxbridge and South Ruislip constituency, Boris Johnson, leave the Advantage children's daycare nursey during a UK general election campaign event in Surbiton, south London on April 22, 2015. Britain goes to the polls to elect a new parliament on May 7. (AFP/Getty Images)
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London Mayor Boris Johnson and Prime Minister David Cameron arrive at the Advantage children's daycare nursery on April 22, 2015 in Surbiton, England. Britain goes to the polls in a General Election on May 7. (Getty Images)
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London Mayor Boris Johnson speaks as Prime Minister David Cameron looks on at an election rally in Hendon on May 5, 2015 in Twickenham, London. Britain will go to the polls in a national election in just two days time. (Getty Images)