Donald Trump went embarrassingly off-script during his state banquet speech, launching a political attack in front of the King in a major breach of protocol.
And in apparently scripted remarks, the US President referred to "free speech" and said the "Union Jack is ever flown" - which many will see as political references to UK domestic politics.
The US President was speaking at a lavish state dinner in his honor at Windsor Castle, marking his second state visit to the UK.
Speaking publicly for the first time since arriving in the UK, Trump apparently veered away from his prepared remarks, turning to the King and saying: "We are, as a country, as you know, doing unbelievably well."
Attacking his predecessor Joe Biden, he went on: "We had a very...sick country one year ago, and today I believe we’re the hottest country anywhere in the world.
"In fact, nobody’s even questioning it...but we owe so much of that to you."
It’s considered against diplomatic protocol for a visiting head of state to make partisan or controversial political remarks during a speech at a state banquet - particularly when the monarch is present.
The President went on to make what could be seen as coded references to claims made by members of his administration that the UK is cracking down on freedom of speech.
And he made an apparently pointed reference to the union flag being "ever flown" - which could have been a reference to the current trend of flags being flown on lamp-posts which has divided opinion in the UK.
"The British empire laid the foundations of law, liberty, free speech and individual rights virtually everywhere," Trump said.
"The Union Jack is ever flown, including in a place called America."
Trump also described the Princess of Wales as "so radiant and so healthy".
And he said: "His Majesty has also raised a remarkable son in His Royal Highness, the Prince of Wales. Really amazing.
"We’ve got to know you and I think you’re going to have an unbelievable success in future.
"Melania and I are delighted to visit again with Prince William and to see Her Royal Highness, Princess Catherine, so radiant and so healthy, it’s so beautiful. It’s really a great honor, thank you."
In his speech, the King joked about nearly being "married off" to the Nixon family. Charles was referring to an occasion in 1970 when, then 21, he spent time with then-president Richard Nixon’s daughter Tricia at social occasions including a dinner and a baseball game.

World Affairs Correspondent