Barack Obama arrived at Downing Street for talks with David Cameron, amid controversy over the US President's forthright support for Britain's continued membership of the EU.
Obama, who earlier enjoyed lunch with the Queen at Windsor Castle, used a newspaper article to argue that EU membership "magnified" the UK's status on the global stage.
His intervention in the bitter EU referendum debate led to claims of hypocrisy from Brexit campaigners, who had urged him to "butt out" of the UK's decision on June 23.
Writing in the Daily Telegraph, the president highlighted the special relationship between his country and the UK "forged as we spilled blood together on the battlefield".
Obama was greeted with a warm handshake from Cameron as he arrived in Downing Street in his armoured limousine - nicknamed "the beast".
The two leaders smiled and waved to photographers on the doorstep of Number 10, but did not respond to reporters' questions about the strength of the US-UK special relationship.
"As citizens of the United Kingdom take stock of their relationship with the EU, you should be proud that the EU has helped spread British values and practices - democracy, the rule of law, open markets - across the continent and to its periphery," wrote Obama, visiting the UK for what is expected to be the last time before he leaves office in January.
"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."
The president said the deaths of tens of thousands of Americans in the battles to liberate Europe showed the strength of the ties between the US, UK and the continent.
"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," he said.
"The 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 as well."
Obama's much anticipated intervention came as a boost to the Remain camp, but infuriated senior figures in the Brexit movement.
Brexit-backing Boris Johnson said Obama "would not dream" of involving the US in an organisation like the EU and accused him of being "downright hypocritical".
But the London mayor came under fire after suggesting in an article in The Sun that the "part-Kenyan" president may be motivated by an "ancestral dislike of the British empire".
Shadow chancellor John McDonnell accused him of "dog whistle racism", while the former British permanent representative to the EU Sir Stephen Wall said the comment was "demeaning to the debate".
Johnson said: "For the United States to tell us in the UK that we must surrender control of so much of our democracy - it is a breathtaking example of the principle of do as I say but not as I do.
"It is incoherent. It is inconsistent, and yes it is downright hypocritical. 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?"