Obama's visit comes two months before the so-called Brexit vote. He's been criticized by advocates of the exit for injecting himself into the domestic British debate.
U.K. Independence Party leader Nigel Farage said Obama should "butt out." And London Mayor Boris Johnson, a leader of the exit campaign, said Americans "would never contemplate anything like the EU for themselves."
Writing in The Sun newspaper, Johnson said Obama's stance "is a breathtaking example of the principle of do as I say, not as I do.