Barack Obama's two-day visit could prove to be a critical moment in the lead-up to the Brexit vote after the US President spoke out in favour of continued UK membership of the European Union.
Here we look at what Obama said during a joint press conference with Prime Minister David Cameron, and how some Brexiters responded, ahead of the June 23 referendum.
The US President defended his right to have an opinion on the vote and said leaving the EU would push the UK to "the back of the queue" in trade dealings with the US
"In democracies everybody should want more information, not less, and you shouldn't be afraid to hear an argument being made - that's not a threat, that should enhance the debate," Obama said.
"Particularly because my understanding is that some of the folks on the other side have been ascribing to the United States certain actions we will take if the UK does leave the EU - they say for example that 'we will just cut our own trade deals with the United States'.
"So they are voicing an opinion about what the United States is going to do, I figured you might want to hear from the president of the United States what I think the United States is going to do.
"And on that matter, for example, I think it's fair to say that maybe some point down the line there might be a UK-US trade agreement, but it's not going to happen any time soon because our focus is in negotiating with a big bloc, the European Union, to get a trade agreement done.
"The UK is going to be in the back of the queue."
Obama admitted the US had a "deep interest" in the outcome
"The United States wants a strong United Kingdom as a partner and the United Kingdom is at its best when it is helping to lead a strong Europe," he said.
"It leverages UK power to be part of the European Union. I don't believe the EU moderates British influence in the world, it magnifies it."
Obama said the UK's membership of the EU "enhances the special relationship" because it meant the US had a trusted partner in Europe on issues including the fight against terrorism.
"Precisely because I have a confidence in the UK, and I know that if we are not working effectively with Paris or Brussels then those attacks are going to migrate to the United States and to London, I want one of my strongest partners in that conversation," he said.
"So it enhances the special relationship, it does not diminish it."
Obama said that the UK's membership of the EU added to collective prosperity and security
"All of us cherish our sovereignty - our country is pretty vocal about that - but the US also recognises that we strengthen our security through our membership of Nato, we strengthen our prosperity through organisations like the G7 and the G20," he said.
"I believe the UK strengthens both our collective security and prosperity through the EU."
Obama rejected a description of the EU as being "in crisis", but said it was "under strain"
"I am confident that the ties that bind Europe together are ultimately much stronger than the forces that are trying to pull it apart," he said.
"When you think about the 20th century and the 21st century, 21st-century Europe looks an awful lot better, and I think a majority of Europeans recognise that.
"They see that unity and peace have delivered sustained economic growth, reduced conflict, reduced violence, enhanced the quality of life of people, and I'm confident that can continue.
"But I do believe it is important to watch out for some of these fault lines that are developing."
What was the response of some Brexiters?
In a statement issued by main Brexit campaign Vote Leave, Justice Minister Dominic Raab focused on Obama's comments on the migration crisis rather than his warning about a trade deal.
"The president made clear that uncontrolled immigration into the EU is a threat to national security," he said.
"I agree - that is why it is safer to take back control so that we can stop terror suspects from Europe coming into the UK.
"He argued that he thinks it is in America's interests for the UK to stay in the EU but what is good for US politicians is not necessarily good for the British people. We want more international cooperation after we Vote Leave, but the EU is not fit for purpose, and cannot cope with the multiple crises we face like terrorism, Syria and mass migration.
"The US would not dream of opening its border with Mexico, so it is hypocritical for President Obama to insist that we do the same with Europe."
Responding to Obama's comments, the co-founder of the Leave.EU campaign, Richard Tice, said: "We don't have a trade deal with the United States now because we're members of the European Union.
"The proposed EU-US trade deal, TTIP (Transatlantic Trade and Investment Partnership), would be disastrous for British workers.
"Obama doesn't have the authority to deny us a deal, as he will be long gone before any such proposals are on the table."