U.S. President Barack Obama made an impassioned appeal on Friday for Britain to remain in the European Union, saying membership had magnified Britain's place in the world and made the bloc stronger and more outward looking.
Fearful that a British exit could weaken the West, Obama arrived in London to applaud Britain's EU membership which he said had helped make the world freer, richer and better able to tackle everything from Russian aggression to terrorism.
Praising Britain's "outsized" influence in the world, Obama invoked the interlinked history of the two countries and the tens of thousands of Americans lying in European war graves as his reason for speaking as "a friend" on the June 23 referendum.
"The European Union doesn't moderate British influence — it magnifies it," he wrote in an article placed on page 20 of the eurosceptic Daily Telegraph newspaper under the headline "As your friend, I tell you that the EU makes Britain even greater."
"The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic".
Obama is due to meet U.S. embassy personnel and families before a lunch at Windsor Castle with Queen Elizabeth, who celebrated her 90th birthday on Thursday, and her husband Prince Philip.
Obama is scheduled to hold talks afterward with Prime Minister David Cameron, followed by a news conference.
Campaigners for Britain's EU membership, including Cameron, who is leading the "In" campaign, will welcome Obama's intervention, which led news broadcasts on British television.
But the president's comments 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."
Cameron has said that this is no time for Britain to drop out of the club it joined in 1973, especially in the face of what he terms Russian President Vladimir Putin's aggression.
Asked about Obama's views, Cameron previously told the parliament, "Personally I believe we should listen to advice from friends and other countries and I struggle to find the leader of any friendly country who thinks we should leave."
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.
For Britain's closest ally, EU membership amplifies British influence, facilitates trade for U.S. companies and strengthens the 28-member bloc that Washington views as a pillar of stability in the post-World War Two era.
Opponents of the EU, many of whom laud the U.S. alliance, have said that membership has shackled Britain to the corpse of a failed German-dominated experiment in European integration, and that Britain, if freed, could prosper as a sole trader.
Nigel Farage, a prominent opponent of membership as leader of Britain's UK Independence Party, called Obama the most anti-British American president to date.
"This is an unwelcome interference … Mercifully, he won't be in office for much longer."