Obama's blunt warning on cost to Britain of leaving EU

US President Barack Obama has been blunt about the economic consequences of Britain voting to leave the European Union. Photo / AP

A trade deal between Britain and the United States could take five to 10 years to negotiate if Britain votes to leave the European Union at a June 23 referendum, US President Barack Obama says.

"It could be five years from now, 10 years from now before we're actually able to get something done," Obama told the BBC.

Obama, who is in the last nine months of his presidential term, has spent the past three days in London urging Britons to remain part of the EU as a divided British public prepares to vote on whether to remain a member of the 28-country bloc.

He told the BBC that Britain would not get preferential treatment over the EU when it came to negotiating a new trade deal.

"The UK would not be able to negotiate something with the United States faster than the EU," Obama said.

"We wouldn't abandon our efforts to negotiate a trade deal with our largest trading partner, the European market."

Obama's visit and decision to intervene in the EU debate has angered the Eurosceptic "Leave" campaign, which has repeatedly argued that Britain could easily negotiate deals and get better terms outside the EU.