US Secretary of State John Kerry said on Tuesday that Britain's vote to leave the European Union might never be implemented and that London is in no hurry to go.

Speaking one day after talks with Prime Minister David Cameron, Kerry said the outgoing British leader feels "powerless" to negotiate a departure he does not want.

"This is a very complicated divorce," Kerry told the Aspen Ideas Festival, referring to Britain's negotiated exit from the EU after last week's "Brexit" referendum.

Kerry, who visited Downing Street on Monday, said Cameron was loath to invoke "Article 50" of the Lisbon treaty, which would trigger a two-year timetable for departure.

London, Kerry explained, does not want to find itself boxed in after two years without a new association agreement and to be forced out of the EU without one.

Asked by the panel moderator if this meant the Brexit decision could be "walked back" and if so how, Kerry said: "I think there are a number of ways."

"I don't as secretary of state want to throw them out today. I think that would be a mistake. But there are a number of ways," he said.

Washington has long supported a strong role for its British ally in the European project, and was dismayed when British voters chose last week to quit the Union.

Now, US officials are calling for a calm debate on Brexit leading to a deal that would allow a close association between London and Brussels to continue.