Britain's Prime Minister David Cameron (R) and US President Barack Obama (L) attend a press conference at the Foreign and Commonwealth Office in central London on April, 22, 2016 © Andy Rain / Reuters

"It could be five years f-rom now, 10 years f-rom now before we were able to actually get something done,” Obama told the BBC on Sunday, at the end of a three-day visit to the UK, likely to be his last as US president.

The UK referendum on whether the country should stay or go will take place on June 23. The Transatlantic Trade and Investment Partnership (TTIP) deal it threatens remains on the president’s agenda before his term runs out. Obama wants to cut tariffs and regulatory barriers between the EU and the US.

Britain would move “to the back of the queue” on deals if it left, Obama said on Friday, at the start of the visit. The UK works best when “helping to lead” the uni-on, and its membership makes it a “bigger player” globally, Obama said.

Read more

‘Back of free trade queue’: Brits slam Obama for ‘threats’ over Brexit

Britain would not be able to negotiate a deal with the US faster than the EU, Obama said, adding that the US “wouldn’t abandon our efforts to negotiate a trade deal with our largest trading partner, the European market.”

This echoed his earlier words at a joint press conference with British Prime Minister David Cameron. "The UK is at its best when it's helping to lead a strong European Uni-on. It leverages UK power to be part of the EU. I don't think the EU moderates British influence in the world, it magnifies it,” he said. "America wants Britain's influence to grow, including within Europe."

Cameron agreed with much of what Obama said: "This is our choice – nobody else's – the sovereign choice of the British people. But as we make that choice, it surely makes sense to listen to what our friends think."

Some US politicians share Obama’s view that a Britain within the EU is able to exert more authority and apply more pressure to push through policies in line with American interests. The loudest voice in support is Democratic presidential hopeful Hillary Clinton.

Read more

Why Obama really wants Britain to stay in the EU… to impose TTIP on Europe – NGO

But the outgoing US president’s comments angered many in the 'Out' campaign in Britain, which believes the UK could strike deals just as effectively outside the European bloc as it could within. Leading Brexit campaigners London Mayor Boris Johnson and UKIP chief Nigel Farage weighed in with criticism of Obama’s intervention.

Johnson called Obama’s comments “hypocritical,” issuing the following comment earlier: "We haven't had a trade deal with America for 43 years. Why not? Do you know why not? Because we're in the European Uni-on.

"The crucial point is that I'm a big fan of Barack Obama - I was one of the first people to come out in favour of him ages ago,” Johnson said on a visit to his constituency in Uxbridge, according to the BBC.

"But I think there's a weird paradox when the President of the Unites States, a country that would never dream of sharing its sovereignty over anything, instructs or urges us politely to get more embedded in the EU, which is already making 60 percent of our laws,” he added.

"I think the issue really is about democracy – America guards its democracy very jealously and I think we should be entitled to do so as well."

Read more

Thousands rally in Hannover against TTIP trade deal a day before Obama’s visit

Opposition to the proposed TTIP deal between the EU and the US continues, with the latest seen in the German city of Hannover as thousands gathered to protest. Anti-TTIP campaigners are strongly against the pro-big business slant of the treaty, believing it would drive down wages and weaken labor rights and consumer protection.

“Stop TTIP!” an estimated 30,000 people changed as they marched peacefully.

While the Transatlantic Trade and Investment Partnership (TTIP) between the US and Europe is set to cre-ate the world's largest free trade zone, many Europeans worry that the agreement would elevate corporate interest above national interest. TTIP opponents say that cheaper goods and services would only hurt the EU and help the US.

Obama’s view is the diametrical opposite of this. The US president is currently on a visit to the German city for the trade fair there.