“I think it’s fair to say that maybe some point down the line there might be a U.K.-U.S. trade agreement, but it’s not going to happen any time soon because our focus is in negotiating with a big bloc, the European Union, to get a trade agreement done,” he said. “The U.K. is going to be in the back of the queue.”

It was the strongest warning yet from Obama about the cost of a British exit from the EU—one that British commentators described as “battlefield nuclear device” against the Leave campaign.

This by Obama is devastating for Leave. He's using his platform to rip the Out case apart — Jonathan Freedland (@Freedland) April 22, 2016

Earlier, writing in The Telegraph, Obama made the case for Britain’s continued EU membership.

“The European Union doesn’t moderate British influence,” he wrote. “It magnifies it.” And, he added: “The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic.”

Polls show the race is close—and the debate heated. Indeed, London Mayor Boris Johnson, a vocal advocate of leaving the EU—the so-called Brexit—appeared to suggest in the Sun Obama’s view had to do with the “part-Kenyan President’s ancestral dislike of the British empire.” It’s a statement for which he’s receiving much criticism—but the crux of his argument centers on the fact it’s a bit rich for the U.S. to lecture Britons to stay in an international institution that, in the London mayor’s view, erodes the country sovereignty, while the U.S. is adamantly opposed to doing anything remotely similar. Here’s more:

For the United States to tell us in the UK that we must surrender control of so much of our democracy – it is a breathtaking example of the principle of do-as-I-say-but-not-as-I-do. It is incoherent. It is inconsistent, and yes it is downright hypocritical. The Americans would never contemplate anything like the EU, for themselves or for their neighbours in their own hemisphere. Why should they think it right for us?

Obama’s response to that criticism was simple: “All of us cherish our sovereignty … but the U.S. also recognizes that we strengthen our security through our membership of NATO, we strengthen our prosperity through organisations like the G7 and the G20. I believe the U.K. strengthens both our collective security and prosperity through the EU.”

Other supporters of Britain’s exit from the EU also criticized Obama’s remarks, some pointing out that it was unseemly for a foreign leader to intervene so overtly in a country’s internal affairs. (Polls show many Briton share that view). But the BBC points out that while historically the two countries have enjoyed a special relationship, the message Obama is sending Britons is the same as the one the U.S. sent after World War II: that the U.S. sees Britain’s place in the world as part of the European project.