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)
