LONDON • United States President Barack Obama has plunged into Britain's poisonous European Union membership debate, arguing strongly against a "Brexit" as he kicked off a visit to the country.

His intervention yesterday - ahead of the June 23 vote - in a piece for The Daily Telegraph newspaper drew a furious response from eurosceptics such as London Mayor Boris Johnson and UK Independence Party leader Nigel Farage, who said he should "butt out".

Writing in the daily - a traditional bastion of euroscepticism - Mr Obama argued that Britain's place in the EU magnified its global influence and was a matter of "deep interest" to the US.

"I realise that there's been considerable speculation - and some controversy - about the timing of my visit," he 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."

A spokesman for British Prime Minister David Cameron welcomed Mr Obama's intervention.

"When you see allies and partners coming out around the world advocating the case for (Britain) remaining in the EU, talking about how that amplifies our influence in the world, we think those are important arguments to hear," said the spokesman.

Mr Obama and US First Lady Michelle Obama yesterday flew by helicopter to Windsor Castle, where they had lunch with Queen Elizabeth II, who turned 90 on Thursday.

Mr Obama's intervention - an unusual foray into the domestic politics of another country - drew withering criticism from "Brexiteers".

Mr Johnson, the leading face of the eurosceptic campaign, said it was "downright hypocritical" of the US to intervene in the debate.

"For the US to tell us in (Britain) 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.

Politics and international relations professor Richard Whitman from the University of Kent said it was "an unusually personal intervention" from Mr Obama.

"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."

While experts noted that many people have not yet decided how to vote, the "Remain" camp currently has 54 per cent support compared with 46 per cent for the "Leave" camp, according to an average of the last six opinion polls by academics at the What UK Thinks project.

Seen from Washington, Mr 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," Mr 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 US, he said.

From Britain, Mr Obama was to travel to Germany for a meeting with German Chancellor Angela Merkel and other EU leaders.

AGENCE FRANCE-PRESSE, REUTERS