From showing solidarity with Volodymyr Zelensky to inviting US President Donald Trump for an historic state visit, Britain's diplomatic drive over Ukraine has a surprising pivotal figure: King Charles III.

The UK head of state may be politically neutral, but that has not stopped the Labour government from calling upon him three times in recent days to aid international diplomacy efforts.

Charles helped smooth Prime Minister Keir Starmer's visit to the White House last week before separately welcoming Zelensky and Canadian leader Justin Trudeau to his country retreat in Norfolk.

"It's slightly unusual, but I think it's a very good use of the royals," professor Pauline Maclaran at Royal Holloway University of London said of the flurry of meetings.

"Their big asset is this soft power that they can provide, and I think it was much needed at this time," the royal expert told AFP.

The 76-year-old monarch appears all-too-happy to play his part, as Britain's diplomatic blitz over Ukraine sees it emerge from the wilderness of the post-Brexit years to take its place again on the world stage.

"It has been six days of royal diplomacy at its most delicate, deliberate and nuanced," a royal source briefed UK media on Monday.

The source added that Charles "is very conscious of his responsibility globally, regionally and nationally -- and passionately engaged in all the detail".

"As a global statesman and a head of state for both the UK and Canada, the king's role is highly significant, and his majesty is determined to play his part, within appropriate parameters," the source added.

Buckingham Palace never shares information about what is discussed during the monarch's meetings, but the source said his role "is to offer symbolic gestures, rather than express comment".

The symbolism was plain to see in the Oval Office last week when Starmer brandished a signed letter from Charles inviting Trump to become the first leader in history to undertake a second state visit to Britain.

In front of the TV cameras a beaming US president read the invitation, said it would be an "honour" to accept, and declared the king "a beautiful man, a wonderful man".

- 'Masterstroke' -

That set the tone for a friendly meeting between Starmer and Trump that was devoid of any verbal fireworks.

"It was a little bit of a masterstroke," said Maclaran, adding that the king's invitation was "to impress Trump, to show him the greatest respect, and to ease the way for Starmer to negotiate with him".

Evie Aspinall, director of the British Foreign Policy Group think-tank, agrees that the gesture helpfully played to Trump's ego.

"He wants to feel big and important, and we can do that" with a state visit, she told AFP.

Charles's next diplomatic endeavour was much less flashy but sent an equally strong message, the analysts say.

On Sunday, he warmly welcomed Zelensky to his Sandringham Estate in eastern England just two days after Trump and Vice President JD Vance berated Ukraine's leader in the Oval Office.

According to royal watchers it cleverly symbolised solidarity with British ally Zelensky, but in a low-key way so as not to offend the thin-skinned Trump.

"The royal family was giving their legitimacy to Zelensky at a time where he really needs it with Trump," said Aspinall.

For Maclaran, it helped portray "equal respect" to both leaders at a time when Britain is trying to bridge the divide between the United States on one side, and Europe and Ukraine on the other.

Charles is Canada's head of state and Trudeau said he was going to use Monday's meeting to talk about defending Canada's sovereignty, as Trump repeatedly calls for it to become the 51st US state.

It is custom that the monarch acts only on the advice of the prime minister in matters involving the Commonwealth nation but some Canadians have asked why the king has not spoken out in defence of Canada.

Royal commentator Richard Fitzwilliams says Charles must remain wary of becoming too involved in diplomacy due to the unpredictability of the Trump era.

"It's not only the complexities, it's the speed at which it moves. It's very confusing and he's got to be very, very careful," Fitzwilliams told AFP.

AFP

King Charles welcomed Zelensky to the Sandringham Estate just two days after the Ukrainian leader's dressing down in the Oval Office © Joe Giddens / POOL/AFP

Diplomat Charles: UK king's role in Trump-Ukraine tightrope act