President Obama support David Cameron's effort to remains in the E.U.
Boris Johnson and Nigel Farage says he's anti-British because he's half Kenyan.
Speaking at a joint press conference with Mr Obama, Prime Minister David Cameron said being a member of the EU strengthened Britain's "special relationship".
On the UK's upcoming referendum on its EU membership, he said: "This is our choice - nobody else's - the sovereign choice of the British people - but as we make that choice, it surely makes sense to listen to what our friends think."
Mr Obama said: "The UK is at its best when it's helping to lead a strong European Union. It leverages UK power to be part of the EU. I don't think the EU moderates British influence in the world, it magnifies it.
"America wants Britain's influence to grow, including within Europe."
On whether he should be intervening on the UK's referendum, he said: "Let me be clear: ultimately this is something the British voters have to decide for themselves."
"As part of our special relationship, part of being friends is to be honest and to let you know what I think, and speaking honestly, the outcome of that decision is a matter of deep interest to the US, because it affects our prosperity as well."
Mr Obama's intervention into the EU referendum debate has been criticised as "hypocritical" by London Mayor and EU leave campaigner Boris Johnson, and UKIP leader Nigel Farage said he was the most anti-British US president ever.
The London mayor has been under fire over comments he made about Mr Obama's "part-Kenyan" ancestry in an article for the Sun, suggesting some people thought he had an "ancestral dislike of the British Empire".
UKIP leader Nigel Farage earlier told BBC News he thought Mr Obama's views on the UK had been shaped by his Kenyan grandfather's experience of colonial Britain and suggested he bears a grudge against Britain.
http://www.bbc.com/news/uk-36115138
