Barack Obama has said the US and UK will have to do everything they can to make sure strong trade links do not start "unravelling" as a result of Brexit.
In a blow to Brexiteers' hopes of a quick trade deal with the US, he stood by his comment that the UK was now at the "back of the queue" for trade talks.
Speaking at the G20 Summit alongside Theresa May, he said the US trade negotiations with the EU was his priority.
Mr Obama said: "We have a lot of investment, including British companies in the United States and US companies in the United Kingdon.
"That's not going to stop. We're going to do everything we can to make sure that the consequences of the [Brexit] decision don't end up unravelling what is already a very strong and robust economic relationship."
Mr Obama gave a warm welcome to Mrs May at her first big international summit, but the meeting was tainted by Mr Obama's intervention during the referendum campaign in which he said the UK would go to the "back of the queue" for trade talks.
When he made the intervention it was hailed by Remainers as proof of the damaging results of a potential vote to quit the EU, but it outraged Leave campaigners who accused him of unduly meddling in British politics.
Boris Johnson, now Foreign Secretary, claimed the Americans would never have stood for being part of something like the European Union themselves.
Challenged over whether he still stood by the remark in front of Mrs May, he defended it and went on to set out that it was still the case.
The US President said negotiations on the controversial Transatlantic Trade and Investment Partnership were ongoing and could not be set aside.
He said: "It is absolutely true that I believe pre-Brexit vote and post-Brexit vote that the world benefitted enormously from the United Kingdom's participation in the EU.
"But I also said at the time [of making the comment] that ultimately this was a decision for the British people."
Mr Obama said that he never meant to say that the US would "punish Great Britain", but simply that he wanted to challenge the "notion" that the consequences of Brexit were negligable and that Brexiteers would "just go ahead and light-up a whole bunch of free trade agreements."
Mrs May said she had used the meeting with the President to discuss Britain's decision to leave the EU, the Brexit process and what it means to the UK's relationships with European states and other nations.
She said: "The UK has always been a strong partner for the US and that will remain to be the case. We have a thriving economic relationship.
"British business export twice as much to the United States as they do to our next largest market and the United States is the largest inward investor in Britain with total investments providing more than one million jobs."
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