Barack Obama has urged Britain to stay in the EU as he arrived in the country for the start of a four-day visit.
Writing in the Daily Telegraph, he said being part of the European Union "enhances Britain's global leadership".
He highlighted the special relationship between the US and the UK "forged as we spilled blood together on the battlefield" saying: "It is a matter of deep interest to the United States.
"The path you choose now will echo in the prospects of today's generation of Americans as well."
But he added: "In this complicated, connected world, the challenges facing the EU - migration, economic inequality, the threats of terrorism and climate change - are the same challenges facing the United States and other nations."
However, his intervention in the bitter EU referendum debate has not gone down well with everyone.
Brexit campaigners, including Boris Johnson, urged him to "butt out" of the UK's decision on June 23.
Johnson said Obama "would not dream" of involving the US in an organisation like the EU and accused him of being "downright hypocritical".
Writing in The Sun 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?"
Ukip leader Nigel Farage added: "President Obama should butt out. This is an unwelcome interference from the most anti-British American president there has ever been.
"Mercifully, he won't be in office for much longer."