The US President has arrived in London for talks with British PM David Cameron
US President Barack Obama has arrived in London and immediately triggered a political row by urging the UK to stay in the European Union.
In a move that will enrage Eurosceptics but delight pro-EU campaigners, the US President says the EU makes Britain even greater.
Writing in The Daily Telegraph, Mr Obama said the sacrifice of his country's soldiers during the Second World War means America has a stake in the referendum debate.
His column read: "Our special relationship was forged as we spilled blood together on the battlefield. From the ashes of war, those who came before us had the foresight to create the international institutions and initiatives to sustain a prosperous peace: the United Nations and NATO; Bretton Woods, the Marshall Plan, and the European Union."
And making his controversial plea, the President said: "The European Union doesn't moderate British influence - it magnifies it. A strong Europe is not a threat to Britain's global leadership; it enhances Britain's global leadership.
"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic. So the US and the world need your outsized influence to continue - including within Europe."
But Mr Obama is already facing a furious backlash from senior Conservative Eurosceptics.
Writing in The Sun, London Mayor Boris Johnson - who is campaigning for the UK to leave the EU - said of his intervention: "It is incoherent. It is inconsistent, and yes it is downright hypocritical."
In his column, Mr Johnson added: "The Americans would never contemplate anything like the EU, for themselves or for their neighbours in their own hemisphere. Why should they think it right for us?"
But Alan Johnson, chairman of Labour In for Britain, said the President's remarks showed it is "no longer credible" to claim the UK could have a closer relationship with the US outside of the EU.
He said: "President Obama is Head of State in a country that has been Britain's ally in war and in peace. US soldiers lost their lives in two world wars on our continent".
"Not only does this give the President an entitlement to comment, I believe he has an obligation to point out the wider ramifications of a British withdrawal from the EU".
President Obama's comments came ahead of a lunch at Windsor Castle with the Queen and Prince Philip to mark the Queen's 90th birthday.
The President and his wife Michelle were greeted by the Queen and her husband on the grass outside the castle.
Later President Obama will attend a dinner hosted by Prince William and his wife Kate.
Meanwhile, countering the terrorist threat from Islamic State is set to dominate the agenda when British Prime Minister David Cameron and Mr Obama meet for talks.
The upcoming 'Brexit' referendum on 23 June will also be discussed.