LONDON — The mayor of London has a simple message for President Obama over Britain’s upcoming referendum on membership in the European Union: Butt out.

In a strikingly combative article published Monday, the flamboyant mayor, Boris Johnson, accused the United States of “exorbitant hypocrisy” in urging the United Kingdom to vote to stay in the bloc, given that the former defends its own sovereignty with “hysterical vigilance.”

Johnson's strident criticism in the Telegraph newspaper comes amid reports that Obama will visit the United Kingdom next month and will urge Britons to vote to remain in the 28-nation E.U. The London mayor is on the other side of the fence, campaigning for severed ties with the bloc ahead of the June 23 referendum.

Impolitic commentary is being flung back and forth across the Atlantic these days, marking a sharp turn from the usual amicability. As my colleague Adam Taylor noted, Obama suggested in a recent interview that British Prime Minister David Cameron was something of a foreign policy headache. Needless to say, the comments sparked front-page headlines in Britain.

Johnson isn’t leaping to Cameron’s defense, though — indeed, they are on opposite sides in the E.U. referendum debate — but his comments are sure to add to the ill winds gusting across the Atlantic.

The relationship between the United States and Britain is famously a “special” one, and it is unusual for politicians to publicly lash out at their counterparts nearly 4,000 miles away. But Johnson is no stranger to controversy. In the article, he argued that Americans would never allow a similar loss of sovereignty as Britain is forced to accept under its E.U. membership.

The North Atlantic Free Trade Agreement, or NAFTA, doesn’t come with a commission, a parliament and a court of justice, he pointed out.

“So why is it essential for Britain to comply with a system that the Americans would themselves reject out of hand? Is it not a blatant case of 'Do as I say, but not as I do?'" he wrote.

He also said that Obama will soon arrive in the U.K. “like some deus ex machina, to pronounce on the matter. Air Force One will touch down; a lectern with the presidential seal will be erected.”

“The British people will be told to be good to themselves, to do the right thing,” Johnson continued. “We will be informed by our most important ally that it is in our interests to stay in the EU, no matter how flawed we may feel that organisation to be. Never mind the loss of sovereignty; never mind the expense and the bureaucracy and the uncontrolled immigration.”

Obama's position on the referendum is no mystery. Unlike in the run-up to the referendum on Scottish independence, in which the White House’s intervention was low-key and last-minute, Obama has been forthright in asserting that the United States believes the United Kingdom would be a stronger ally if it stayed in the E.U.

Those campaigning for Britain to remain in the bloc welcomed a direct intervention from Obama.

Foreign Secretary Philip Hammond said that although the British people would decide the referendum, they should listen to the views expressed by international leaders.

"I think it's important that we hear from those people in the Anglosphere — not just President Obama but the leaders of Australia, New Zealand, Canada — and beyond the Anglosphere, Japanese and Chinese leaders. Let's just hear what they actually think about their relations with Britain,” Hammond told reporters Monday.

It’s unclear what effect a direct intervention from Washington could have on voting behavior. The pollster YouGov is asking Britons that question, with results expected later Monday afternoon.

But those campaigning to leave the E.U. are aware that Obama remains a popular figure in their country, and they arguably wouldn’t be telling him to stay out of the E.U. debate if they didn’t think he could make a difference in what is shaping up to be a tight contest.