US President Barack Obama has made an impassioned appeal for Britain to remain in the European Union, saying membership had magnified the country'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 tell Britons that issues such as terrorism, migration and economic slowdowns could be tackled more successfully with the UK in the EU.
In approaching such a divisive issue, he 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 the Daily Telegraph newspaper on Friday (local time).
"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 scheduled to have lunch on Friday at Windsor Castle with Queen Elizabeth, who celebrated her 90th birthday on Thursday, and her husband Prince Philip. He is to hold talks after lunch with Prime Minister David Cameron.
Obama's visit is a welcome one for Cameron, who is leading the "In" campaign, but it has drawn scorn from those arguing that Britain should leave the bloc.
Opinion polls indicated that British voters are leaning towards the "In" camp, but many remain undecided.
The US government, and many US banks and other companies, fear a Brexit would cause market turmoil, diminish the clout of its strongest European ally, undermine 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, adding that ultimately the referendum was a matter for British voters to decide.
The comments will be welcomed by Cameron who 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.
Opponents of the EU, many of whom laud the US 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.
"Out" campaigners have said the US would never agree to dilute its own national sovereignty in the way the EU requires of its member states.