The U.S. ambassador to London has publicly warned British Prime Minister Theresa May to side with President Donald Trump in the burgeoning transatlantic dispute over the controversial nuclear deal with Iran, which the U.S. leader withdrew from in May.



Ambassador Woody Johnson cautioned there would be trade consequences for Britain, which he described as the closest U.S. ally, unless it breaks with the European Union and follows Trump in re-imposing sanctions on Tehran. The envoy also delivered a clear ultimatum to British businesses, instructing them to stop trading with Iran or face "serious consequences" when it comes to trade with the United States.



The unprecedented warning, which was delivered in an article published by Britain's Sunday Telegraph newspaper, is being seen in London as the opening shot in what could be the biggest test of the so-called "special relationship" between the United States and Britain since Trump took office.



Trump's decision in May to withdraw from the 2015 nuclear deal, signed by his predecessor Barack Obama, in which Tehran agreed to nuclear curbs in return for sanctions relief, paved the way for the restoration of unilateral American economic penalties on Iran.

U.S. President Donald Trump holds up a proclamatio FILE - U.S. President Donald Trump holds up a proclamation declaring his intention to withdraw from the JCPOA Iran nuclear agreement after signing it in the Diplomatic Room at the White House in Washington, May 8, 2018. FILE - U.S. President Donald Trump holds up a proclamation declaring his intention to withdraw from the JCPOA Iran nuclear agreement after signing it in the Diplomatic Room at the White House in Washington, May 8, 2018.

While ratcheting up pressure on Tehran, the sanctions are worsening rifts between the United States and European allies, and other world powers, which say they remain committed to the nuclear deal, and will not comply with the U.S. sanctions.



In the article, Johnson said the Trump administration is "determined to make sure they [the sanctions] are fully enforced." He added, "The President has been explicit: any businesses that put their commercial interests in Iran ahead of the global good will risk serious consequences for their trade with the U.S."



He added, "America is turning up the pressure and we want the UK by our side. We are asking global Britain to use its considerable diplomatic power and influence and join us."



A British Foreign Office official said Sunday, "We remain committed to the nuclear deal. But we have had discussions with Washington about how we can work together in other ways to curb activity by Iran in the Middle East which concern us."



A week ago Britain signed on to a joint statement with other EU countries that pledged to press on with a strategy to lessen the impact of the U.S. sanctions on European businesses. It includes prohibiting them from complying with the unilateral U.S. sanctions.



Asked if British companies could expect Britain to stand its ground over deal, a British minister, Alistair Burt, told reporters last week, "They can expect us to do that, yes. Sometimes you need to take a stand against friends."



The U.S. sanctions that began last week prohibit any transactions with Iran involving dollars, gold, precious metals, aluminum, steel, commercial passenger aircraft, shipping and Iranian seaports.



The U.S. administration blames Iran for fomenting instability in the Middle East and encouraging terrorism. Trump has described the 2015 nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), as a "horrible, one sided" agreement.

In his article Sunday, Johnson said, "Only by presenting a united front can we exert the maximum possible pressure on the Iranian regime and get them to finally change course and put an end to their malign and reckless activities both at home and abroad."



Johnson said Tehran had used money going into the country after the 2015 deal and the easing of sanctions not to improve the lives of ordinary Iranians but to increase spending on the military and proxy forces in the Middle East, including sponsoring Hezbollah in Lebanon, arming militants in Yemen, and launching cyber-attacks against Western democracies.



Britain's May might face a party leadership challenge later in the year, possibly from Boris Johnson, the former foreign minister who says she should be more like Trump. She is struggling to quell rebellions within the ranks of her Conservative party over Brexit negotiations and she can't afford to alienate Brussels further by siding with Washington on the Iran nuclear deal, say analysts.



On several tempestuous issues dividing Trump and Europe in a deepening rift, May has been caught trying to please both sides. In June during the acrimonious G-7 meeting, held in Charlevoix, Quebec, which broke up amid highly personal recriminations between Trump and fellow summiteers over trade tariffs, May appeared especially eager to keep a low profile. Of all the G-7 leaders at the bruising summit, she largely side-stepped the public skirmishing.



She recorded her disappointment, but avoided leveling personal criticism.



May's position is likely to become even more awkward in the coming days as she's forced to choose between European leaders, whose support she needs for a favorable Brexit deal or a U.S. leader determined to secure EU compliance with reimposed sanctions on Iran, fear British officials.