US secretary of state expresses doubts over whether Brexit will happen as David Cameron ‘has no idea how he would do it’

The US secretary of state has raised doubts about whether Brexit will ever happen, suggesting most leave campaigners do not truly believe in Britain’s divorce from the EU and do not know how to achieve it.

Claiming there were a number of ways in which Thursday’s vote could be “walked back”, John Kerry, who visited Downing Street on Monday, said David Cameron was loth to invoke article 50, the EU exit procedure.

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He said the British prime minister felt powerless to “start negotiating a thing that he doesn’t believe in” and “has no idea how he would do it”.

Apparently referring to Boris Johnson, one of the frontrunners to replace Cameron, Kerry added: “And by the way, nor do most of the people who voted to do it.”

Cameron was worried that Britain would be forced out of the EU at the end of the two-year negotiating period without a trade deal, Kerry told Aspen Ideas festival on Tuesday. Asked 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.”

The US was a big cheerleader of Britain’s continued EU membership and has stood by White House warnings that the UK would go the “back of the queue” when trade deals were being negotiated.

Earlier in the week, Kerry stressed the rupture with the EU would have consequences, saying it was “not possible” the vote would have no impact.

If the British government invokes article 50, it will enter into two years of negotiations on an EU withdrawal treaty. EU officials have described the process is “irreversible” once launched, although that is not stated in the treaty text. Legal experts have told the House of Lords that a country could change its mind, albeit with “substantial political consequences”.

The article 50 talks would cover Britain’s EU exit, including the status of EU nationals living in the UK and Britons on the continent. A trade deal would be negotiated separately, a process that insiders think would take anywhere from five to 10 years. The EU side insists Britain cannot have a trade deal until the article 50 divorce is signed and sealed.

European leaders have voiced frustration at Britain’s delay in triggering article 50, urging Cameron’s successor to act speedily. Officials are angry and incredulous that during the campaign, leave politicians “didn’t have a clue” what they wanted.

“What I don’t understand is that those who want to leave are totally unable to tell us what they want,” Jean-Claude Juncker, the president of the European commission, said after Cameron’s final EU summit on Tuesday. “I thought that if you wanted to leave you had a plan ... they don’t have it.”