US President Barack Obama has told an audience of young British people to reject isolationism, pessimism and cynicism.
Speaking after he made it clear he does not want the UK to leave the EU, he appeared to make a veiled call for young people to vote to stay in the EU during an event billed as a US-style 'town hall' meeting.
The Remain campaign got a further boost on Saturday night as the favourite to win the Democratic nomination in the race for the White House, Hillary Clinton, reportedly made clear that she was in favour of the UK staying in the EU.
Her senior policy adviser Jake Sullivan told the Observer: "Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united.
"She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU."
Mr Obama told his audience at Royal Horticultural Halls on Saturday: "We see calls for isolationism or xenophobia. We see those who would call for rolling back the rights of people.
"I think we can understand they are reactions to changing times.
"But, when I speak to young people, I implore them, and I implore you, to reject those calls to pull back."
But Out campaigner Boris Johnson, who was criticised on Friday for an attack on Mr Obama described as racist, shrugged off the president's criticism and continued to claim he was being "hypocritical".
"I think this is all a complete distraction. An attempt by the Remain campaign to throw dust in people's eyes," he said.
"Over the last few days, nobody on that side of the argument has been able to answer the key point that I have been making which is that it is inconsistent, perverse and yes, it is hypocritical of the United States to tell us that we should sacrifice more of our independence than they would ever dream of doing themselves."
Mr Obama's message on Friday, made as he stood alongside Prime Minister David Cameron, had been one of warning as he suggested a trade deal between the UK and US would be at the "back of the queue" if the UK quit the bloc.
But it was an altogether more positive tone he took on Saturday.
"My primary message is going to be to reject pessimism and cynicism, know that progress is possible and that problems can be solved," the President said.
"Progress requires the harder path of breaking down barriers and building bridges, and retaining the values of tolerance that our nations have worked to defend."
Justice Minister Dominic Raab branded the President an irrelevant "lame duck" after his remarks.
"I have got no doubt that future US trade negotiators are going to look to other opportunities - I think the British will be first in the queue, not at the back of the queue."
Earlier, UKIP leader Nigel Farage also savaged Mr Obama's comments, saying the President's intervention was at the "bidding of Cameron" and accused him of "doing his best to talk down to Britain".
On Friday night, Mr Obama and First Lady Michelle had dinner with the Duke and Duchess of Cambridge and Prince Harry, where they also met Prince Georg e.
During the 'town hall' meeting, the President said that George was adorable .
Before his meeting on Saturday, Mr Obama visited the Globe Theatre to mark the 400th anniversary of William Shakespeare's death.
He then went on to have talks with Labour leader Jeremy Corbyn, who said afterwards that he had an "excellent" discussion with the President, with subject matter including inequality and the impact of technology and global corporations on world populations.
Tonight, the President will meet David Cameron again, when US ambassador Matthew Barzun will host a private dinner.