"Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united. She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU," Clinton senior policy adviser Jake Sullivan said in a statement provided to Reuters.
Trade deal foes plan major German rally on eve of Obama visit
Clinton, who spent four years as US secretary of state during Obama's first term, has made her foreign policy credentials a central piece of her campaign for president in the November 2016 race for the White House.
Clinton's position on the EU was first reported by Britain's Observer newspaper.
Obama, who will leave Britain on Sunday, sparked a row during his three-day trip by bluntly telling Britain it should remain in the European Union to preserve its remaining global clout.
Obama issues stark trade warning against Brexit
He angered critics of the EU on Friday by warning that Britain would be at "the back of the queue" for a trade deal if it left the club - one of the strongest US interventions in the affairs of a western European democracy since the Cold War.
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