Obama and his wife Michelle flew by helicopter on Friday to Windsor Castle, a royal residence that traces its history back over almost 1,000 years to William the Conqueror, for lunch with Queen Elizabeth, and her husband, Prince Philip.

The queen celebrated her 90th birthday on Thursday.

Obama will later hold talks with Prime Minister David Cameron, followed by a news conference.
The president's comments on the EU drew scorn from opponents of Britain's EU membership.
New York-born London Mayor Boris Johnson, who heads the "Out" campaign, said that he did not want to be lectured by Americans about EU membership and that the United States would never countenance such a transfer of sovereignty.

"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," Johnson wrote in the Sun newspaper.

"It is incoherent. It is inconsistent, and yes it is downright hypocritical," Johnson said.
Opinion polls indicate that British voters are leaning towards the "In" camp but many remain undecided. "In" campaigners are concerned that young voters may not turn out to vote.

The U.S. government, and many U.S. banks and companies, fear a Brexit would cause market turmoil, diminish the clout of its strongest European ally, hurt London's global financial hub status, cripple the EU and weaken Western security.

"Now is a time for friends and allies to stick together," Obama said. "Together, the United States, the United Kingdom, and the European Union have turned centuries of war in Europe into decades of peace, and worked as one to make this world a safer, better place."

"Unwelcome Interference"

Ahead of a 2014 Scottish vote on independence, Obama said he hoped Britain "remains strong, robust and united," a comment that was welcomed by unionist politicians in London.

Citing President Franklin D. Roosevelt's 1939 toast to Washington's close alliance with Britain, Obama lauded Britain's contribution to the development of democracy, the rule of law and open markets.