The US President described one of the newest royals, the Duchess of Sussex -- the former Meghan Markle -- as "nasty" when he was told the American had once described him as "misogynistic."
"I didn't know that she was nasty," Trump replied in an interview with "The Sun" newspaper. He then tweeted Sunday morning: "I never called Meghan Markle 'nasty.'"
And Trump waded into the Conservative Party's contest to find a new Prime Minister and Britain's paralyzing debate on leaving the European Union, in a way sure to outrage British critics.
Most presidents would go out of their way to avoid such sensitive topics at a moment of extreme political stress. In Trump's case they may deepen his already intense unpopularity in Britain ahead of his arrival for a three-day stay on Monday but enhance his global reputation as an unpredictable, disruptive influence.
Respecting diplomatic niceties has never been Trump's style, and his remarks underscored the intense challenge his visit poses to the "special relationship" between the US and Britain.
Trump's incendiary remarks were conveyed in a pair of bombshell interviews with the "Sun" tabloid and the "Sunday Times" -- papers owned by Rupert Murdoch, the proprietor of Fox News. They came as Britain prepares trademark pomp for Trump, who will be guest of honor at a state banquet hosted by Queen Elizabeth II at Buckingham Palace on Monday, designed to indulge his taste for adulation. First lady Melania Trump will accompany the president to the opulent ballroom.
The President has a habit of ignoring the political and diplomatic sensitivities of his hosts during trips abroad. In Japan just last week, for instance, where he was greeted with supreme flattery and royal ceremony, the President indicated he wasn't much bothered by North Korea's missile tests. His comments did not take into account the fact that such behavior is viewed with alarm and a grave security threat by the government in Tokyo.
Trump's comments ahead of his trip to Britain will also come as a new blow to beleaguered Theresa May, whose premiership has been destroyed by her failure to solve the political crisis provoked by Brexit.
May will trigger a Conservative Party leadership election Friday which will begin a search for her successor and likely produce a new prime minister by the end of the summer. Trump's Washington is clearly rooting for a more openly Euro-skeptic successor to May who could provide an ideological partner more closely aligned with the current White House.
UK has 'got to get it done'
In the Sunday Times interview, Trump suggested that May would have been better off had she adopted his barnstorming negotiating style in intricate exit talks with the EU.
He said May should refuse to pay the $49 billion divorce payment required by the EU if Brussels does not give into Britain's demands and said she should sue the European bloc.
"They've got to get it done," Trump told the paper. "They have got to get the deal closed."