Rocker Rod Stewart has appeared to call US President Donald Trump a “little ratbag” during an exchange with the King.

The 81-year-old Maggie May singer greeted the King in a receiving line at an event in London on Monday (local time) alongside Ronnie Wood from the Rolling Stones.

Stewart did not name Trump but told the King he did a great job on his state visit to the US in April.

“May I say, well done in the Americas,” Stewart says in a clip of the exchange.

“You were superb, absolutely superb… put that little ratbag in his place.”

The King’s back is to the camera and his response cannot be clearly heard in the video, although he can be seen gesturing with his hands.

The Independent reported that the monarch gave a joking reply.

Stewart listened to the King’s part of the conversation and then continued.

“Exactly, that’s it – it went right over his head, right over his head.”

Later, Stewart makes a similar remark to the Queen.

“I was just congratulating your husband on his wonderful performance in the Americas, so great, so brave, so proud,” he says.

Stewart, who was once friends with Trump and lived in Florida, has not been quiet about his changed feelings towards the President.

The legendary singer shared his opinions in January, calling Trump a “draft dodger” and demanding he apologise for criticising troops in Afghanistan.

“I may just be a humble rock star. I’m also a knight of the realm, and I have my opinions. I was born just after the war (World War II), and have great respect for our armed forces that fought and gave us our freedom,” he said.

“It hurts me badly, deeply, when I read that the draft dodger Trump has criticised our troops in Afghanistan for not being on the front line.

“We lost over 400 of our guys. Think of their parents. Think about it! And Trump calls ’em almost like cowards. It’s unbearable.

“So I’m calling on you, Prime Minister [Keir] Starmer and [Reform UK leader Nigel] Farage. Please, make the draft dodger Trump apologise, please.”

The King and Queen with Trump at the White House during their visit. Photo: AAP

The King’s state visit to Washington in April was hailed a success for helping to stabilise the UK-US relationship.

Much was made of the personal relationship between the King and Trump. A senior palace aide involved in the trip said the two men “get on very well”, and “the warmth that you see in public is absolutely the warmth you see in private”.

“They get on very well. And it’s not just the King and the President. It’s all four of them with each other,” they said.

“It’s a pairing, sort of unlikely though it might be in many ways … I mean, given some of the issues that have presented themselves in the bilateral relationship, you’d think it might all be a bit tricky. But far from it.

“The warmth that you see in public is absolutely the warmth you see in private.”

When the King and Trump sat down in the Oval Office for their bilateral meeting, there was lots of “laughter” alongside the serious talk.

Republicans and Democrats rose to their feet 12 times to applaud during the King’s speech in Congress’ Hall of the House, where he spoke of the importance of NATO, support for Ukraine, the value of European partners and the need to reduce world conflicts, all priorities for Britain.

“Beneath King Charles’s jokes and decorum, some subtle rebuttals to Trump, King Charles III gently pushed back against President Trump’s attacks on Britain and NATO, and he spoke of the importance of checks and balances,” The New York Times said.

The King’s Congress address has been widely interpreted as a gentle push back against some of Trump’s pet peeves — Ukraine, NATO, the need to follow rules and the climate change lobby.

But he was guided by the “truth” and his “conscience” when delivering his speech that found favour with many Republicans and Democrats, and its contents were “a measure of how much he personally cares”.

The monarchy’s soft power diplomacy is seen as its strongest weapon in support of Britain’s overseas interests and it appears Trump was won over during the four days, repeatedly praising the “fantastic” King.

-with AAP