The UK could take up to 10 years to negotiate trade deals with the US if it leaves the EU, Barack Obama has said.
In a BBC interview, the US president said: "It could be five years from now, 10 years from now before we were able to actually get something done."
Britain would also have less influence globally if it left, he added.
His warning over UK-US trade deals has angered campaigners for leaving the EU - with Mayor of London Boris Johnson calling his comments "hypocritical".
Labour MP Gisela Stuart, who co-chairs Vote Leave, said it was "extraordinary" the US should urge Britain to remain part of a "dysfunctional organisation".
On Friday, Mr Obama said the UK would be at the "back of the queue" for trade deals with the US, if it left the EU.
Asked about the comments, he told the BBC: "The UK would not be able to negotiate something with the United States faster than the EU.
"We wouldn't abandon our efforts to negotiate a trade deal with our largest trading partner, the European market."
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Mr Obama will leave the UK later and travel to Germany for talks with Chancellor Angela Merkel.
The BBC's correspondent in Berlin said the main aim of the visit was to win public support in Germany for the planned US-EU free trade deal.
During a wide-ranging interview, Mr Obama said:
- It would be "a mistake" for any Western state to send ground troops into Syria
- Achieving a functioning state in Libya had "enormous ramifications" for Europe
- Countries could not "pull up the drawbridge" when faced with the migration crisis
- The operation against so-called Islamic State would "slowly shrink" the area they operate in
- His wife, Michelle, was a remarkable woman, and on parenting: "I am labour, she's management"
His fresh intervention in the EU debate comes as Democratic US presidential hopeful Hillary Clinton also made it clear she thought it would be a mistake for the UK to leave the union.
Her senior policy adviser Jake Sullivan said: "Hillary Clinton believes that transatlantic co-operation is essential, and that co-operation is strongest when Europe is united.
"She values a strong British voice in the EU."
The Vote Leave campaign said Mrs Clinton's comments were "hollow", but a Downing Street source said "we should listen to our closest friends and allies".
The UK's referendum on whether the country should stay in the EU or leave will take place on 23 June.
'More influence'
Returning to the UK's place in the EU, Mr Obama said the US wanted the UK to be "at the table" influencing countries that might not see things from a US view.
Despite criticism from pro-Brexit campaigners, he said he thought "ordinary British voters" would be interested in his thoughts on the UK and the EU.
The US president warned the UK would have "less influence in Europe and as a consequence, less influence globally", if it left the EU.
"Since we rely heavily on the UK as a partner globally on a whole range of issues, we'd like you to have more influence," he told the BBC.
His intervention in the EU referendum has drawn fierce criticism from pro-Brexit campaigners.
Ms Stuart said: "What I think is so extraordinary is that an American president should urge us to join [sic] an utterly dysfunctional organisation.
"We should shape the future by leaving an organisation that is neither capable of reforming nor changing."
London Mayor Mr Johnson said the UK could be in a more favourable position to strike a trade deal with the US if it left the EU.
'Best mother I know'
Mr Obama was also asked about his priorities for the final nine months of his presidency.
He mentioned implementing the Paris Agreement on climate change, prosecuting the campaign against so-called Islamic State, and concluding a trade agreement with the EU.
Mr Obama is pushing hard for a proposed Transatlantic Trade and Investment Partnership (TTIP) deal, to cut tariffs and regulatory barriers between the US and EU countries.
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AP
The president also praised the First Lady, his wife, Michelle.
"I cannot separate anything that I've achieved from the partnership that I've had with that remarkable woman," he told the BBC.
"First and foremost, she's the best mother I know, and my daughters are amazing because of her parenting.
"I am labour, she's management, you know, so I can take very little credit beyond executing some of the plans that she has."
'Reject pessimism'
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Kensington Palace
Mr Obama's visit to the UK is likely to be his last as US president.
On Saturday, he visited the Globe theatre where he watched actors perform scenes from Hamlet as part of events to mark the 400th anniversary of the death of Shakespeare.
Speaking in London, he urged young people to "reject pessimism and cynicism" and "know that progress is possible and problems can be solved".
On Friday, Mr and Mrs Obama visited the Queen at Windsor Castle and held talks with Mr Cameron, before dining with the Duke and Duchess of Cambridge and Prince Harry at Kensington Palace.