Hillary Clinton has warned Britain against leaving the European Union

White House candidate Hillary Clinton has also waded into the Brexit debate to warn Britain against leaving the European Union. The presidential front-runner is the second American heavyweight to voice opinion on the upcoming referendum, after Barack Obama intervened at a press conference in London last week. The former secretary of state's senior policy adviser, Jake Sullivan, told The Observer: 'Hillary Clinton believes that transatlantic cooperation is essential, and that co-operation is strongest when Europe is united. Read more: