President Obama has made a dramatic intervention in the Brexit debate, claiming it is in the "deep interest" of the United States that the UK remain in the European Union.
His long exepected comments have angered pro-Brexit campaigners, including Boris Johnson, who argue the president should not interfere in internal-UK domestic politics.
Writing in The Daily Telegraph today, Obama said the US-UK special relationship which was "forged as we spilt blood together on the battlefield" is helped by Britain staying in the EU.
Invoking the wartime alliance, Obama added: "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."
The president's column was published as he landed in Britain for a short visit in which he will meet the Queen and hold talks with David Cameron.
The prime minister and president are due to hold a joint press conference in which Obama is expected to repeat his pro-EU argument.
Cameron hopes Obama's popularity in the UK will help convince the British public to vote to remain inside the EU at the June 23 referendum.
Obama said: "Now is a time for friends and allies to stick together.
"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. 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."
"Today, we face tests to this order – terrorism and aggression; migration and economic headwinds – challenges that can only be met if the United States and the United Kingdom can rely on one another, on our special relationship, and on the partnerships that lead to progress.
In a direct response to Obama, Johnson, writing in The Sun, said the UK would be an "even better and more valuable" ally to the US outside the EU.
"The US guards its democracy with more hysterical jealousy than any other country on earth," he 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?"
In his article, Obama admitted there has been "some controversy" about his visit.
Yet he added: "But also I understand that there’s a spirited campaign under way here. My country is going through much the same. And ultimately, the question of whether or not the UK remains a part of the EU is a matter for British voters to decide for yourselves.
"So 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. 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."