The Queen warmly greeted Donald Trump at a glittering event at Buckingham Palace tonight.
But the US President arrived late to mingle with other members of the British royal family, including a giggling Kate Middleton.
Mr Trump and First Lady Melania had to cut short their to dash to meet the Queen at Buckingham Palace.
But and helicopters hovered above as the president's motorcade arrived at Clarence House 40 minutes late.
Both the duchess and Mrs Trump opted for bold colours, with Camilla wearing a red dress under a black cape jacket and the First Lady dressed in a £4,700 Valentino coat with fuchsia heels.
Kate chose an Emilia Wickstead high-neck crepe midi dress, believed to be worth £965.
World leaders including Turkey's President Recep Tayyip Erdogan and Prime Minister of Canada Justin Trudeau also attended the glittering function.
But Mr Trump, 73, had to rush to leave them so he could attend his dinner meeting with Boris Johnson at Downing Street.
The hundreds of NHS workers joined the anti-Trump protest from Trafalgar Square to Buckingham Palace, bearing signs that read “Trump hands off our NHS” and “cut war not health care”.
It came hours after Trump vowed he wasn't interested in the health service, saying he wouldn't take it even on a "silver platter".
But protesters turned up in force to show they didn’t believe him, or Boris Johnson’s claims the NHS would be protected.
Charles earlier met other Nato leaders, including Prime Minister of Canada Justin Trudeau, Prime Minister of Norway Erna Solberg and Prime Minister of Italy Giuseppe Conte.
Ms Solberg put the environment on the agenda as she gave the prince a football decorated with sustainability goals.
She told him it is used to get young people to talk about the issue.
"I don't know if you play football," she joked as she gave the gift.
Charles replied: "A very, very long time ago."
He threw the ball to a royal aide as he took Ms Solberg out of the morning room.
After the Trumps were last welcomed for tea at Clarence House on the president's state visit to the UK in June, Mr Trump revealed he was pressed on climate change by Charles.
Mr Trump, who has previously accused climate experts of having a "political agenda", told ITV's Good Morning Britain he "totally listened" to the prince when the issue came up.
He said: "What he really feels one way about is the future.
"He wants to make sure future generations have climate that is good climate as opposed to a disaster, and I agree.
"I did mention a number of things, I did say the United States right now has among the cleanest climates."
Mr Trump landed at Stansted Airport for his latest visit late on Monday night.
His visit comes as he faces a fight for his political life back at home and pressure from Prime Minister Boris Johnson not to meddle in the general election campaign.
The Nato summit will conclude on Wednesday afternoon.
