Is the US president’s outburst to the Sun simply bad manners, or his latest attempt to undermine an old ally?

He once likened his relationship with Theresa May to that between Ronald Reagan and Margaret Thatcher. Presumably that comparison is now cancelled.

Donald Trump has once again torn up standard etiquette for diplomacy by turning up, not with flowers or a bottle of wine – but a verbal grenade. As he was setting off for his first visit to the UK as American president, he told the Sun that he advised Theresa May “how to do” Brexit but “she didn’t listen to me”.

He also warned that a soft Brexit will probably kill any hope of a separate US-UK trade deal, which was supposed to be a centrepiece of Friday’s talks at Chequers. And to add insult to injury, he suggested that May’s nemesis, Boris Johnson, would “make a great prime minister”.

Play Video 1:14 Donald Trump says Sun interview is 'fake news' – video

Clearly, when in 2016 Trump declared himself “Mr Brexit”, he should have been taken both seriously and literally. His ego and belief in his own deal-making skills are such that he apparently thought he alone could fix it. Few observers believe that he has studied the vast mounds of paperwork or the complex web of laws involved.

His outburst to the Sun – a Eurosceptic tabloid newspaper owned by Rupert Murdoch, whose Fox News channel supplies many of Trump’s views and staff – could certainly be seen as bad manners, perhaps an act of revenge for the baby blimp set to take to the London sky on Friday.

Trump: soft Brexit will 'kill' UK's chances of US trade deal Read more

On another, equally Trumpian level, it might be regarded as his latest brazen attempt to undermine an old ally. Just as the president tore into Angela Merkel’s Germany over pastries and cheese before the Nato summit had even started, now he has May’s Britain in his sights.

Play Video 0:46 'I think they like me a lot in the UK', says Donald Trump – video

He is aware that May, like Merkel, is weakened and vulnerable domestically, and his past record suggests that he despises weakness. He has consistently expressed admiration, by contrast, for dictators such as Vladimir Putin, Kim Jong-un and Rodrigo Duterte.

More than a year into his presidency, no one is quite certain whether this is due to an instinctive fascination with autocrats and the great man theory of history – or a deeper, more sinister effort to reorder the world in favour of rightwing demagoguery.

How Trump's Nato summit meltdown unfolded Read more

For now, it means that Trump and May’s engagements – a joint forces military demonstration, a working lunch at Chequers and a joint press conference – promise the height of social awkwardness. Watch for the handshake – always a tell with Trump.

Based on past, self-contradictory form, the president, when confronted by reporters, will probably seek to play down his negative comments about a bilateral trade deal, triggering a fresh set of headlines about how it might be back on.

May, meanwhile, standing before the media and live TV cameras, will face calls to emulate Hugh Grant’s prime minister in the 2003 film Love Actually, who informs the American president: “A friend who bullies us is no longer a friend. And since bullies only respond to strength, from now onward I will be prepared to be much stronger.”

Failing that, as Trump goes on to take tea with the Queen, the prime minister could always join the demonstrations on the streets of London. It seems she has nothing to lose.