President Barack Obama flew out of Britain Sunday after warning that it could take a decade for the U.K. to negotiate a new trade agreement with the United States if it leaves the European Union.
In a BBC interview, Obama said "it could be five years from now, 10 years from now before we were able to actually get something done."
The U.S. and the 28-nation European bloc of which Britain is a member are attempting to seal a trade deal, the Trans-Atlantic Trade and Investment Partnership, or TTIP.
Obama said that "the U.K. would not be able to negotiate something with the United States faster than the EU."
"Our preference would be to work with this large block of countries," he added.
Obama flew to Germany after a three-day U.K. visit during which he angered supporters of an EU exit by making it clear the United States wants Britain to stay in.