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: