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US President Barack Obama (L), Britain's Prince William, Duke of Cambridge (R) US First Lady Michelle Obama (2L), Catherine, Duchess of Cambridge (L) and Britain's Prince Harry (C) sit together in a reception room at Kensington Palace in London, April 22, 2016 where the Obamas were hosted. Barack Obama warned Britain against leaving the European Union on Friday, undercutting a key argument of eurosceptics by saying London would be 'at the back of the queue' for a post-Brexit trade deal. / AFP / POOL / STEPHEN CROWLEY (Photo credit should read STEPHEN CROWLEY/AFP/Getty Images)

US President Barack Obama (L), Britain's Prince William, Duke of Cambridge (R) US First Lady Michelle Obama (2L), Catherine, Duchess of Cambridge (L) and Britain's Prince Harry (C) sit together in a reception room at Kensington Palace in London, April 22, 2016 where the Obamas were hosted. 
Barack Obama warned Britain against leaving the European Union on Friday, undercutting a key argument of eurosceptics by saying London would be 'at the back of the queue' for a post-Brexit trade deal. / AFP / POOL / STEPHEN CROWLEY        (Photo credit should read STEPHEN CROWLEY/AFP/Getty Images)