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LONDON, ENGLAND - APRIL 22: Prince William, Duke of Cambridge speaks with US President Barack Obama as Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama and Prince Harry in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England. The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

LONDON, ENGLAND - APRIL 22: Prince William, Duke of Cambridge speaks with US President Barack Obama as Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama and Prince Harry in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England.  The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

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LONDON, ENGLAND - APRIL 22: Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England. The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

LONDON, ENGLAND - APRIL 22: Catherine, Duchess of Cambridge speaks with First Lady of the United States Michelle Obama in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England.  The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

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LONDON, ENGLAND - APRIL 22: In this handout provided by The White House, President Barack Obama talks with the Prince William, Duke of Cambridge as Catherine, Duchess of Cambridge plays with Prince George as First Lady Michelle Obama talks with Prince Henry at Kensington Palace on April 22, 2016 in London, England. The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union.(Photo by Pete Souza/The White House via Getty Images)

LONDON, ENGLAND - APRIL 22:   In this handout provided by The White House, President Barack Obama talks with the Prince William, Duke of Cambridge as Catherine, Duchess of Cambridge plays with Prince George as First Lady Michelle Obama talks with Prince Henry at Kensington Palace on April 22, 2016 in London, England.  The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union.(Photo by Pete Souza/The White House via Getty Images)

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LONDON, ENGLAND - APRIL 22: Prince William, Duke of Cambridge speaks with US President Barack Obama in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England. The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

LONDON, ENGLAND - APRIL 22: Prince William, Duke of Cambridge speaks with US President Barack Obama in the Drawing Room of Apartment 1A Kensington Palace as they attend a dinner on April 22, 2016 in London, England.  The President and his wife are currently on a brief visit to the UK where they attended lunch with HM Queen Elizabeth II at Windsor Castle and later will have dinner with Prince William and his wife Catherine, Duchess of Cambridge at Kensington Palace. Mr Obama visited 10 Downing Street this afternoon and held a joint press conference with British Prime Minister David Cameron where he stated his case for the UK to remain inside the European Union. (Photo by Dominic Lipinski - WPA Pool/Getty Images)

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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)