President Barack Obama leaned in further on his warning to the British electorate against embracing a so-called Brexit from the European Union, saying it could take as long as 10 years before the U.K. and the U.S. negotiated a new trade agreement.

“The U.K. would not be able to negotiate something with the United States faster than the EU,” Obama told the BBC in an interview broadcast on Saturday. “We wouldn’t abandon our efforts to negotiate a trade deal with our largest trading partner, the European market. But rather, it could be five years from now, 10 years from now before we were able to actually get something done.”

The president’s remarks were the most detailed he’s made yet in addressing the argument that withdrawing from the EU would hurt the British economy by delaying its ability to sign a trade deal with the U.S. Obama, who said Friday that Britons would be at the “back of the queue” in negotiating a trade agreement with the U.S. separately from the EU.

The comments extended the rare intervention of a U.S. president into another nation’s domestic politics. On Friday, Obama stood beside Prime Minister David Cameron to admonish the British electorate about the perils of embracing an isolationist stance.

Cameron invited Obama to visit the U.K., about two months before British voters choose whether to terminate their membership in the EU in a vote set for June 23.

Clinton’s Backing

Support for the U.K.’s EU membership also came from the campaign of Hillary Clinton, the frontrunner for the Democratic presidential nomination.  “Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united,” senior policy adviser Jake Sullivan said in a statement published Sunday by the Observer newspaper. “She values a strong British voice in the EU.”

In speaking out against Brexit, Obama drew on the long history of relations between the U.S. and the U.K. But not everyone in Britain has been happy to hear the American president’s opinions.

Brexit backer and London Mayor Boris Johnson wrote in The Sun newspaper that Obama’s intervention was “downright hypocritical.” He suggested the “part-Kenyan” president might dislike Britain’s imperial legacy -- a comment that drew widespread criticism.

“The Americans would never contemplate anything like the EU for themselves or for their neighbors, in their own hemisphere,” Johnson wrote. “Why should they think it right for us?”

Ipsos Mori polling data show the British public split over whether Obama should express his opinion on the June 23 referendum, with 49 percent saying he should and 46 percent saying he shouldn’t. Separately, the bookmaker Ladbrokes Plc on Friday said that 90 percent of all bets taken in the past two days have been for “Remain.” The prospects of a “Leave” vote fell to 29 percent from 34 percent, it said. The Bloomberg Brexit Tracker puts the probability of vote to quit the EU at about 20 percent.

A Pew Research Center poll in June found that 76 percent of Britons have confidence in Obama on matters relating to world affairs, and Ipsos Mori on Friday said 15 percent of Britons say his view on the referendum is important to them in deciding how they’ll vote.

Other polls show that young people largely support the U.K. staying in the EU, but are also less likely to show up to vote than other groups. The referendum will be held while many young people will be attending graduations or the Glastonbury Festival, a five-day music and performing arts event.