London - US President Barack Obama plunged into Britain's increasingly poisonous EU debate on Friday at the start of a visit, warning strongly against Brexit and pointing out that US soldiers had died for Europe.
Obama's intervention ahead of the EU referendum in June drew a furious response from eurosceptics like London Mayor Boris Johnson and UK Independence Party leader Nigel Farage, who said he should "butt out".
Writing in the Daily Telegraph, a traditional bastion of euroscepticism, Obama argued that Britain's place in the EU magnified its global influence and was a matter of "deep interest" to the United States.
"I realise that there's been considerable speculation - and some controversy - about the timing of my visit," Obama wrote.
Stressing that the choice was purely for the British people, he wrote: "I will say, with the candour of a friend, that the outcome of your decision is a matter of deep interest to the United States.
"Tens of thousands of Americans who rest in Europe's cemeteries are a silent testament to just how intertwined our prosperity and security truly are.
"And the path you choose now will echo in the prospects of today's generation of Americans." The issue of Brexit is likely to surface again at talks with Prime Minister David Cameron later on Friday, to be followed by a press conference.
Ahead of the meeting, Obama and First Lady Michelle Obama will lunch at Windsor Castle with Queen Elizabeth II, who turned 90 on Thursday.
Obama's intervention - an unusual foray into the domestic politics of another country - drew withering criticism from "Brexiteers".
Johnson, the leading face of the eurosceptic campaign, said it was "downright hypocritical" of the US to intervene in the debate.
"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," he said in a piece for the Sun, Britain's top-selling tabloid.
Richard Whitman, professor of politics and international relations at the University of Kent, said Obama's was "an unusually personal intervention".
"He's making a very strong appeal from the heart," he said.
"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.
While experts warn many people have not yet decided how to vote, the "Remain" camp currently has 54 percent support compared to 46 percent for "Leave", according to an average of the last six opinion polls by academics at the What UK Thinks project.
Seen from Washington, Cameron's decision to call a referendum was a bold - if not downright risky - gamble that could leave Britain and the EU badly weakened.
"The EU has helped spread British values and practices - democracy, the rule of law, open markets - across the continent and to its periphery," Obama wrote.
"The European Union doesn't moderate British influence - it magnifies it.
Britain's voice in the EU keeps the bloc "outward looking" and "closely linked" to the United States, he said.
Cameron, who is leading the campaign to remain in the EU and fighting for his political life, sought to underscore the close ties between the two powers.
The British leader said his talks with Obama would focus heavily on the fight against the Islamic State jihadist group, one of the many areas of cooperation between Washington and London.
"Britain's relationship with the United States is special and enduring. Based on shared values and convictions it has stood the test of time," Cameron said in a statement.
"I am deeply proud of what it has allowed us to achieve, in dealing with the global challenges we both face." During Obama's visit he and the first lady will also dine Friday with Queen Elizabeth's grandson Prince William, his wife Kate and his brother Prince Harry.
From Britain, he will travel to Germany for a meeting with German Chancellor Angela Merkel and other EU leaders.