US President Barack Obama warned Britain on Friday that it would find itself "at the back of the queue" for a trade deal with the United States if it voted to leave the European Union in a referendum in June 23.
Obama told British voters that their country's influence on the world stage was "magnified" by its membership of the 28-member bloc and that, as a close friend and ally, the US felt a deep interest in the issue.
Speaking at a joint news conference with British Prime Minister David Cameron during a visit to Britain, Obama said Britain's EU membership enhanced the "special relationship" between Washington and London.
"I think this makes you guys bigger players," Obama said.
On trade, he said the US 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.
"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 not because we don't have a special relationship but because given the heavy lift on any trade agreement, us having access to a big market with a lot of countries rather than trying to do piecemeal trade agreements is hugely efficient."
He said that in the event of a Brexit, a US-British trade deal might happen, but it would not be anytime soon.
Obama's robust arguments will be music to the ears of Cameron and others in the "In" camp, but those campaigning for an "Out" vote have accused the US president of meddling.
Opinion polls suggest that "In" is ahead, but the race is tight and the number of undecided voters is very high.
Obama's comments at the news conference followed an opinion article in a British newspaper in which he invoked the interlinked history of the US and Britain and the tens of thousands of Americans lying in European war graves.
"As your friend, I tell you that the EU makes Britain even greater," the headline of Obama's article read.
His remarks, which led television news broadcasts in Britain, undercut one of the most passionate arguments of the opponents of EU membership: that Britain could prosper on an equal basis with global powers such as the US.
Lunch with Queen
Before talks with Cameron in Downing Street, Obama and his wife Michelle congratulated Queen Elizabeth, who celebrated her 90th birthday on Thursday.
Prince Philip, Elizabeth's 94-year-old husband, took the wheel of a Range Rover to drive the Obamas to lunch on the territory of Windsor Castle, a royal residence that traces its history back over almost 1,000 years to William the Conqueror.
Two years ago, ahead of a Scottish vote on independence, Obama said he hoped Britain "remains strong, robust and united," a comment that was welcomed by unionist politicians in London. Scots voted to remain part of the United Kingdom.