[LONDON] Barack Obama warned Britain on Friday against leaving the European Union, undercutting a key argument of eurosceptics by saying London would be "at the back of the queue" for a post-Brexit trade deal.
The US president's comments on the June 23 EU membership referendum at a press conference with British Prime Minister David Cameron drew a furious reaction from those campaigning to leave the 28-country bloc.
Standing alongside Mr Cameron at the Foreign Office in London, Mr Obama said Britain was "at its best when it is helping to lead a strong Europe".
The US president, whose term ends next January, made an unusually detailed and heartfelt intervention in the politics of another country and repeatedly spoke of the "special relationship" between Britain and the United States.
But his most significant remarks in nearly an hour of comments came on trade, reflecting growing concern in Washington at the prospect of Britain leaving the EU.
Asked what would happen if Britain did vote to quit, Mr Obama said that while "maybe at some point" it could seal a trade deal with the United States, "it's not going to happen any time soon".
"The UK's going to be at the back of the queue," Mr Obama added.
Anti-EU campaigners like London Mayor Boris Johnson have made the claim that Britain could sign free trade deals with allies around the world a key plank of their argument for leaving the bloc.
Nigel Farage, leader of the eurosceptic UK Independence Party, dismissed the president's comments.
"President Obama won't be in office by the time we're out of the EU post-referendum," he wrote on Twitter. "Trade deal of course in both countries' interests."
For his part, Mr Cameron restated his case for Britain remaining in the EU, a close fight which will define his political legacy.
"Now I think is a time to stay true to our values and stick together with our friends and allies," he said.
Mr Obama's comments fuelled a controversy ignited earlier Friday by an article he wrote in The Daily Telegraph newspaper.
The president argued that Britain's place in the EU magnified its global influence.
"The outcome of your decision is a matter of deep interest to the United States," he wrote in comments published at the start of his four-day visit.
Mr Johnson, the leading face of the eurosceptic campaign, said it was "downright hypocritical" of the United States to intervene as it would not accept the same limits on its own sovereignty as EU members do.
"For the United States to tell us in the UK that we must surrender control of so much of our democracy is a breathtaking example of the principle of do as I say, not as I do," Mr Johnson wrote in The Sun tabloid.
Mr Johnson also repeated claims that "part-Kenyan" Obama may have removed a statue of Britain's World War II prime minister Winston Churchill from the Oval Office at the start of his first term out of "ancestral dislike of the British empire".
Mr Obama jokingly dismissed the allegation and said he had another bust of Churchill in his residence.
"I love Winston Churchill. Love the guy," he said.
Ahead of the press conference, Mr Obama and First Lady Michelle Obama had lunch at Windsor Castle with Queen Elizabeth II, who turned 90 on Thursday, and her husband Prince Philip.
They later had dinner with the monarch's grandson Prince William, his wife Kate and brother Prince Harry.
Richard Whitman, professor of politics and international relations at the University of Kent, said Mr Obama was "making a very strong appeal from the heart".
"It will be difficult to say from the polls whether his intervention made a significant difference but I think that it creates a narrative which appears to be favouring the 'Remain' campaign," he said.
A Sky News television survey found 57 per cent said Mr Obama's intervention would make "no difference" to their vote.
While experts warn many people have not yet decided how to vote, the "Remain" camp currently has 54 per cent support compared to 46 per cent for "Leave", according to an average of the last six opinion polls calculated by the What UK Thinks project.