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David Cameron, U.K. prime minister, listens during a news conference following a meeting of European Union (EU) leaders in Brussels, Belgium, on Friday, Feb. 19. 2016. European Union leaders reached a deal aimed at keeping the U.K. in the bloc, allowing Cameron to call a referendum on EU membership as soon as June. Photographer: Jasper Juinen/Bloomberg via Getty Images

David Cameron, U.K. prime minister, listens during a news conference following a meeting of European Union (EU) leaders in Brussels, Belgium, on Friday, Feb. 19. 2016. European Union leaders reached a deal aimed at keeping the U.K. in the bloc, allowing Cameron to call a referendum on EU membership as soon as June. Photographer: Jasper Juinen/Bloomberg via Getty Images