The US President used a joint press conference with Theresa May at the G20 summit in Hangzhou, China to warn that while he does not want to “punish” Britain, there is a risk that the trading relationship between the two countries could “unravel”.

Mr Obama also said he “continues to believe” that the world would be more prosperous if Britain remained a member of the EU.

The British Prime Minister today faces a potentially tense meeting with President Xi Jinping of China over Hinkley Point nuclear power station in Somerset.

She will refuse to give Mr Xi assurances about the project and will not reach a decision until later this month.

There are mounting indications that Mrs May is poised to block Hinkley Point amid security concerns about China’s substantial investment.

Read more: UK’s May stands firm on pledge to leave EU

Earlier, Mr Obama said: “It is absolutely true that I believed pre-Brexit vote, and continue to believe post-Brexit vote, that the world benefited enormously from the United Kingdom’s participation in the EU.

“But I also said at the time that ultimately this was a decision for the British people and the British people made that decision.”

He defended his warning at the height of the EU referendum campaign that Britain would go to the “back of the queue” for trade deals. He said it would “not make sense” to put existing negotiations “aside”.

“We are fully supportive of a process that is as little disruptive as possible, so that people around the world can continue to benefit from economic growth.

“What I have committed to Theresa is that we will consult closely with her as she and her government move forward with Brexit negotiations to ensure that we don’t see adverse affects in trade and commercial relationships.

“We’re going to do everything we can to make sure that the consequences of the decision don’t end up unravelling what is already a very strong and robust economic relationship that can become even stronger in the future.”

Meanwhile, Japan warned Britain that its banks, car manufacturers and pharmaceutical companies may leave the UK for Europe if Brexit leads to the loss of tariff-free trade.

A Japanese government task-force has published a 15-page list of demands from companies and warned that Britain may become an increasingly “unattractive” place for business. It suggested that the UK should enable workers to move easily between the UK and Europe.