US Secretary of State John Kerry has reportedly said that Brexit might never happen.

According to Agence France-Press (AFP), Mr Kerry told the Aspen Ideas Festival in Colorado that there were “a number of ways” in which the UK could walk back from Brexit.

Speaking a day after meeting David Cameron in Downing Street, Mr Kerry reportedly said the Prime Minister was very reluctant to invoke Article 50 of the Lisbon treaty, which would trigger a two-year timetable for departure from the European Union.

"This is a very complicated divorce," he was quoted as saying.

According to AFP, Mr Kerry said that he thought Mr Cameron "feels powerless -- and I think this is a fair conclusion –- to go out and start negotiating a thing that he doesn't believe in, and he has no idea how he would do it.”

"And by the way nor do most of the people who voted to do it," Mr Kerry reportedly said, possibly referring to "Leave" campaigners such as former London mayor Boris Johnson.

In the immediate aftermath of the Brexit victory in the EU referendum, Mr Johnson, now considered by many to be the favourite to replace Mr Cameron as Prime Minister, had said there was “no need for haste” in triggering Article 50.

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Mr Kerry, when asked by the panel moderator if a Brexit decision could be "walked back", reportedly told the ideas festival audience it could be done, although he didn’t at this stage want to say how.

"I think there are a number of ways," he was quoted as saying. "I don't as Secretary of State I want to throw them out [publicise them] today. I think that would be a mistake. But there are a number of ways.”

Mr Kerry’s suggestion that Brexit might not happen comes after demonstrations in the UK against Britain leaving the EU and after thousands signed petitions for a second referendum.

UK constitutional lawyers have also said that MPs still had the legal right to vote down any move to trigger Brexit, with Geoffrey Robertson QC telling The Independent the referendum had been “purely advisory”.

EU Commission President Jean Claude Juncker, however, has made clear his view that “it is not an amicable divorce” and Britain should start negotiating exit terms immediately

The Obama Administration has long been clear in its opposition to Brexit. In April, while visiting the UK, Barack Obama said Britain would be at the “back of the queue” in any trade deal with the US if it left the EU.

However, on Tuesday Mr Obama sought to calm diplomatic and financial market fears about Brexit by telling American National Public Radio: “I would not overstate it. There's been a little bit of hysteria post-Brexit vote, as if somehow Nato’s gone, the trans-Atlantic alliance is dissolving, and every country is rushing off to its own corner. That's not what's happening.