Britain's Prime Minister David Cameron (L) and US President Barack Obama (R) wave outside the door to 10 Downing Street | Adrian Dennis/AFP via Getty Barack Obama to UK: ‘Going it alone’ means no quick trade deal US president says he hasn’t come to ‘fix votes’ — but lays out case for staying in the bloc.

LONDON — U.S. President Barack Obama stepped forcefully into the Brexit debate Friday, warning the U.K. that a vote to leave the European Union would send it to "the back of the queue" when it comes to a future trade deal with America.

"The United Kingdom is at its best when it is leading a strong Europe," Obama told a news conference alongside Prime Minister David Cameron, quoting the poet John Donne with the words "no man is an island — even an island as beautiful as this."

"Americans want Britain's influence to grow including within Europe," said the president. "In the 21st century the nations who make their presence felt on the world stage aren't those who go it alone."

'It's not going to happen any time soon because our focus is negotiating with a big bloc, the European Union' — Barack Obama

Obama dealt a blow to one of the main arguments of Brexit supporters that the U.K. would, if it decides to leave the EU in its June 23 referendum, be able to negotiate a quick trade deal.

"I think it's fair to say that maybe at some point down the line there might be a U.K.-U.S. trade agreement," said Obama. "But it's not going to happen any time soon because our focus is negotiating with a big bloc, the European Union, to get a trade agreement done." He was referring to the planned Transatlantic Trade and Investment Partnership (TTIP).

"The U.K. is going to be in the back of the queue," Obama continued.

'I love Winston'

Rebutting charges that he was interfering in U.K. affairs, the president repeatedly said that it was entirely up to British voters whether they want to stay in the EU or not, and he hadn't come "to fix any votes" or make any threats.

Obama’s final visit to the U.K. before departing the White House risked being overshadowed by an unseemly racism row ignited by Conservative London Mayor Boris Johnson’s allegation that the U.S. president had an “ancestral dislike” for Britain because he was “part Kenyan.”

The provocative claim, in an article in the Sun newspaper, sparked accusations from the Labour Party that Johnson’s Brexit campaign had descended to “dog whistle racism” in a bid to win votes. The toxic row exposes the growing hostility on both sides of the EU referendum campaign, eight weeks out from the June 23 poll.

Johnson also attempted to question Obama's fondness of the U.K. by citing a debunked story that when he moved into the White House, he had jettisoned a bust of Winston Churchill. The president, whose administration had already rebutted that story in 2012, said there was a bust of Winston outside the Treaty Room on the second floor of the official residence.

"... I see it every day, including on weekends when I’m going into that office to watch a basketball game, the primary image I see is a bust of Winston Churchill," he said. "It’s there voluntarily, ‘cause I can do anything on the second floor. I love Winston Churchill. Love the guy."

Jewel in the crown

The president arrived at Downing Street after having lunch in Windsor Castle with Queen Elizabeth and Prince Philip. Obama gushed about the Queen, describing her as "truly one of my favorite people. She is an astonishing person and a real jewel to the world."

In their news conference, Obama and Cameron emphasized repeatedly their commitment to the "special relationship" between their two countries, saying they had discussed how to maintain the military squeeze on ISIL, tackle global terror and counter Russian aggression.

However, the president’s call for Britain to remain in the EU dominated their joint news conference. His decision to set out his case against Brexit, first in an article in Friday’s Daily Telegraph, infuriated other Conservative MPs beside Boris Johnson.

Dominic Raab, the justice minister and leading Vote Leave campaigner, lashed out at the president for “demeaning” the U.K. and treating it as “little more than a North Dakota or Alabama.” Energy Minister Andrea Leadsom told POLITICO Obama’s intervention was “deeply regrettable,” adding: “When did we start thinking it was okay to interfere in other people’s elections?”

Cameron aides were privately happy with the controversy, believing it got his intervention even more attention in the media. One of the Number 10's key goals is for Obama's visit to whip up interest among younger voters. Its internal polls suggest the president has approval ratings of 83 percent in the U.K. — and 91 percent with voters undecided about Brexit.