(CNN) With a friend like Donald Trump, Theresa May doesn't need enemies.
The President delivered an astonishing political knifing of the British Prime Minister on Thursday, comprehensively undermining her fragile position in Britain's tortuous negotiations on leaving the European Union and getting his visit to the country off to the most explosive of starts.
In an interview with the Rupert Murdoch-owned newspaper The Sun, Trump said May had ignored his advice on Brexit, he praised former Foreign Secretary Boris Johnson -- who has just walked out of her Cabinet over the issue -- and he said May's cherished hope of a free trade deal with the US would be killed off by her softened approach.
"She didn't listen to me," he told The Sun.
Trump's comments represent a stunning intervention in British domestic politics, especially since they came at the end of a week in which she lost three ministers, including Johnson, who said her approach is a betrayal of a referendum vote to leave the EU.
They are a fresh sign that Trump has no time for diplomatic niceties, is either oblivious to the political pressures that foreign leaders face or simply does not care about them and is willing to sow disruption wherever he goes in order to enhance his own outspoken political brand.
They are also just the latest instance of Trump, who cozies up to foreign strongmen like Russian President Vladimir Putin and North Korea's Kim Jong Un, insulting or criticizing an allied leader.
And they follow his belligerent performance at the NATO summit, where he demanded more defense spending from members and created new divisions in the Western alliance.
For May, Trump's visit could not have got off to a worse start, at a time when her long-term political prospects are in serious doubt owing to a rebellion of some members of Parliament from her Conservative Party over the terms of Britain's departure from the EU.
The President's interview also amounted to a personal rebuke of his host. It broke just as Trump left a lavish black-tie dinner in his honor hosted by May at Blenheim Palace at the start of a visit that presents her with a deeply difficult test given Trump's unpopularity in Britain and her own wobbly political position.
White House press secretary Sarah Sanders tried to put out the blazing controversy by saying Trump was grateful for a wonderful welcome in Britain.
"The President likes and respects Prime Minister May very much. As he said in his interview with the Sun she 'is a very good person' and he 'never said anything bad about her.' "
But Trump's comments on the ultra-sensitive issue of Brexit will boost May's Conservative opponents, who want a complete split with all EU institutions, and will undermine her claims that her plan does honor the hopes of "Leave" voters two years ago.
"The deal she is striking is a much different deal than the one the people voted on," Trump said in the interview, and added that he had told May to toughen up.
"I actually told Theresa May how to do it but she didn't agree, she didn't listen to me. She wanted to go a different route. I would actually say that she probably went the opposite way."
It was already clear that British hopes of a full free-trade deal with the United States could be complicated by May's plan, since it envisages that Britain would observe European rules on goods and agricultural products.
But Trump's frankness causes a huge problem for the Prime Minister, one that British officials will hope can be finessed when the two leaders meet Friday at Chequers, the Prime Minister's country residence west of London.
In her remarks at the dinner on Thursday, May insisted a trade deal between the US and Britain was still possible, apparently oblivious that the President had already undercut her.
"As we prepare to leave the European Union, we have an unprecedented opportunity to do more," May said. "It's an opportunity to reach a free trade agreement that creates jobs and growth here in the UK and right across the United States."
May has tried hard to cultivate a good relationship with the wrecking ball President, in large part because Britain needs strong trading ties with the US more than ever now that it is leaving the EU. But she has paid a price, taking heavy criticism when she was seen holding hands with Trump at the White House in early 2017 hours before he announced a travel ban on residents of mainly Muslim-majority nations that sparked global outrage.
Trump's comments may deepen his unpopularity in the country since they are likely to be seen as an unwanted intervention by a foreign leader in domestic politics. Thousands of people were already expected to flock to central London on Friday to protest his visit.
The interview is unlikely to change many minds about Brexit, which has carved a deep divide in the electorate. But it might spark some sympathy for the embattled Prime Minister.
Trump's endorsement of Johnson must be especially irksome to May. While being careful not to say Johnson should replace her, Trump said: "I'm just saying I think he would be a great prime minister. I think he's got what it takes, and I think he's got the right attitude to be a great prime minister."
Trump's comments to The Sun were all the more surprising since he tried several times earlier on Thursday not to barge into the Brexit debate.
"It's not for me to say what they should be doing in the UK," he said at a news conference at the NATO summit in Brussels, Belgium.
His comments were published too late for much immediate political reaction. But the Mirror tabloid said the President had dealt May a "crushing blow." The Times newspaper said Trump had "savaged" May's Brexit plan.
Trump is not the first American President to get in hot water over comments about Brexit. President Barack Obama warned in 2016 that Britain would have to get to "the back of the queue" for a trade deal with Washington if it went ahead and voted to leave the EU.
At the time, Obama faced a surge of complaints that he was interfering in an issue that was none of his business, and Brexit campaigners said he was asking Britain to cede powers to the EU that he would never allow the United States to give up.
Trump was a vocal supporter of Brexit, and he arrived in Britain in 2016 for a visit to his Scottish golf courses shortly after the referendum result was announced. He equated the vote to the same populist forces that powered his own anti-establishment election campaign.
The President also had harsh words for London Mayor Sadiq Khan, with whom he wages a long-running feud. Khan approved an application by anti-Trump protesters for a permit to fly a blimp over London in the shape of a baby emblazoned with Trump's face.
Trump said Khan had "done a very bad job on terrorism" and a "terrible job in London."
"I think he has done a bad job on crime, if you look, all of the horrible things going on there, with all of the crime that is being brought in."
Trump did have complimentary things to say in The Sun interview about one person: Queen Elizabeth II, who he will meet for tea with first lady Melania Trump at Windsor Castle later on Friday.
"If you think of it, for so many years she has represented her country, she has really never made a mistake. You don't see, like, anything embarrassing. She is just an incredible woman," Trump said.