By Roberta Rampton and Kylie MacLellan
US President Barack Obama has made a bold intervention into the politics of Washington's closest ally, exhorting Britons to stay in the EU and warning that if they left they would be at "the back of the queue" for a US trade deal.
Obama's plea to British voters ahead of a June referendum on membership of the European Union was welcomed by Prime Minister David Cameron and other supporters of the EU on Friday, but denounced by those campaigning to leave as meddling in British affairs.
Britain's influence on the world stage was "magnified" by its membership of the 28-member bloc, Obama said at a press conference alongside Cameron, who has bet his political future by calling the referendum to put to rest an issue that has divided his own Conservative Party for generations.
Rebutting criticism that he was interfering, Obama invoked the cherished "special relationship" between Washington and London.
"If one of our best friends is in an organisation that enhances their influence and enhances their power and enhances their economy, then I want them to stay in it," Obama said. "Or at least I want to be able to tell them: 'I think this makes you guys bigger players.'"
On trade, he took aim at one of the main "Out" arguments - that Britain could easily negotiate deals and get better terms on its own.
The United States would regard a deal with the EU as a higher priority than a separate agreement with a much smaller market such as a stand-alone Britain, Obama said.
"It's fair to say that maybe some point down the line there might be a UK-US trade agreement but that's not going to happen anytime soon because our focus is negotiating with a big bloc, the European Union, to get a trade agreement done," Obama said.
"And the UK is going to be in the back of the queue."
Cameron said Britain should listen to its friends, and he could not think of any close ally who wanted a Brexit.
But those campaigning for an "Out" vote in the June 23 referendum were dismissive.
London's New York-born Mayor Boris Johnson, a leader of the "Out" campaign from within the Conservative Party widely seen as angling for Cameron's job, said Obama's advice was "incoherent, inconsistent and downright hypocritical".
Obama was urging Britain to pool its sovereignty with other nations in a way that the United States would never countenance for itself, Johnson wrote in a newspaper column.
He also referred to "the part-Kenyan President's ancestral dislike of the British empire", a comment widely criticised as demeaning the EU debate, and even denounced as "dog-whistle racism" by an opposition Labour politician.
Opinion polls suggest that "In" is ahead, but the race is tight and the number of undecided voters is very high.