It was another symbolic gesture of support for Canada from a King, wearing a red tie, who has to send coded signals rather than spell things out in words, as Canada faces threats from US President Donald Trump.

But the King has sought to make clear his commitment to Canada – and if it had not been for his cancer diagnosis, the BBC understands he would have travelled there for an intended visit in 2024.

There are also suggestions that once Canada’s election is out the way, a visit to Canada will be a priority, where he can further demonstrate his support.

The new Canadian prime minister told the King that his Order of Canada pin had broken this morning. The King joked: “Do you want another one?”

“There’s much to catch up on,” said the King, ushering Carney to a seat, and perhaps hoping that the broken pin was not a symbol of a Commonwealth relationship under strain.

“These are important matters,” said the King, ahead of a 30-minute conversation with no one else in attendance.

The meeting with Carney is the latest scene in an intricate diplomatic balancing act for a King who is head of state of both the UK and Canada.

King Charles has to show solidarity with Canada without disrupting the UK’s relationship with Trump, who is antagonising Canadians with his economic threats and calls for Canada to become the US 51st state.

The King also has to avoid getting directly involved in politics – and has to speak on advice of ministers. Whatever he might personally believe about Trump’s comments about taking over Canada, the King has to keep his thoughts to himself.

Adding to this complicated choreography is that the royals are one of the strongest cards the UK can play with President Trump. He seemed delighted by his second state visit invitation from the King.

So messages from the King are sent in symbolic displays. In case anyone missed the signs over Canada, there have been multiple moments. And then some more.

When the King visited aircraft carrier HMS Prince of Wales, he appeared wearing a set of Canadian medals.

The 60th anniversary of Canada’s maple leaf flag might usually have passed without any royal intervention, but there was a fulsome message sent by the King, praising the “proud, resilient and compassionate country”.

A ceremonial sword for Canada became an event at Buckingham Palace with a formal presentation by the King.

At a tree planting ceremony in Buckingham Palace, the tree chosen was a maple. And when the King sat down at the Commonwealth Service last week, it was on a Canadian chair.

If any of these moments were accidental, they were not being rejected by Buckingham Palace, with royal sources emphasising the King’s commitment to Canada.

But there is no avoiding the tensions and contradictions in this balancing act. Emails from Canadians to the BBC’s Royal Watch newsletter show that many want a much more robust defence from the King.

“What a cop out! Throw Canada to the wolves. Giving us back pats for our flag’s anniversary doesn’t quite cut it. Here sits a monarchist who is seriously rethinking his allegiance to the crown!” emailed Brian, a Canadian military veteran.

Carol in Vancouver was unhappy at the invitation from the UK for a second state visit by President Trump.

“I feel ashamed for the Brits feeling they have to follow through with inviting such a boor for dinner. For the life of me I do not understand why he has this power over you,” she wrote.

“As a Canadian, the invitation is a slap in the face to the people of Canada. If the King is our King (as we are a Commonwealth country), and Trump is basically at war with us, how dare King Charles give him any credence?” emailed Patricia.

“As a Canadian I am saddened, appalled, disgusted and angry that King Charles appears to be joining the conga line of supplicants,” said Jo-Ann in Ontario.

But King Charles will have to keep to the line set by ministers, and if that means maintaining good relations with Trump, he is unlikely to be saying anything more outspoken.

In the annual message to mark the day on Monday, the King will emphasise the value of nations coming together in a “spirit of support, and crucially, friendship”.

The King has become involved in the diplomatic negotiations involving the UK, the US and Ukraine – with the monarch warmly welcoming President Volodymyr Zelensky after the Ukrainian leader’s disastrous meeting with US President Donald Trump and with a state visit planned for Trump.

A Commonwealth country, Canada, also faces its own tensions with Trump’s administration, as he has called for Canada to become the US 51st state.

Commonwealth Day is traditionally marked with a service at Westminster Abbey and this year the King and Queen will attend alongside the Prince and Princess of Wales.

In his written message, which will be published in full on Monday, the King will say: “In these uncertain times, where it is all too easy to believe that our differences are problems instead of a source of strength and an opportunity for learning, the Commonwealth’s remarkable collection of nations and peoples come together in the spirit of support and, crucially, friendship.”

It is a message that he has made before – that the diversity of peoples and cultures is a strength and not a weakness.

But it is a message now against a backdrop of global tensions, as the US pushes for an end to the war that followed the Russian invasion of Ukraine.

As well as a show of solidarity with the Ukrainian President Zelensky, the King this week held a meeting with Canadian PM Justin Trudeau, a Commonwealth leader who has faced pressure from the US.

King Charles is Canada’s head of state and ahead of the meeting Trudeau said: “Nothing seems more important to Canadians right now than standing up for our sovereignty and our independence as a nation.”

The King used the Commonwealth, with its 56-member nations, as an example of co-operation between countries.

“The Commonwealth’s ability to bring together people from all over the world has stood the test of time and remains as ever-important today,” says the King’s message.

The service at Westminster Abbey on Monday will emphasise the multicultural diversity of the Commonwealth, with the royal guests being met by a pipe band from a Hindu eco-temple in north-west London.

There will be music from an African arts group, the Masai Cultural Arts team.

And singer Joan Armatrading will perform a version of her song, Love and Affection.

An innovation for this year’s Commonwealth Day has been that the King has prepared his own personal playlist of favourite music tracks, by artists including Bob Marley, Kylie Minogue and Raye.

Trump has repeatedly spoken of making the neighbouring country the 51st state of America. He has also vowed to impose new import taxes on Canada, which is one of his top trading partners.

Speaking ahead of his meeting with the King at Sandringham, outgoing PM Trudeau said he would discuss issues of importance to Canadians, including “standing up for our sovereignty and our independence as a nation”.

The King has also extended an invitation to Trump to visit him in what will be an unprecedented second state visit.

As head of state of the Commonwealth nation, the King has faced calls to give Canada his vocal backing in the face of Trump’s statements.

But Charles faced a difficult tightrope to walk, the former leader of the Canadian province of Alberta suggested.

Writing on X, Jason Kenney said the King could only act on the advice of the Canadian PM, who “should ask [Charles] to underscore Canadian sovereignty”.

New import taxes announced by Trump are also set to come into effect on Tuesday, targeting goods arriving from China and Mexico as well as Canada.

The president is eager to protect American manufacturing and jobs, and to address the US trade deficit.

He suggested in a post on his Truth Social platform on Sunday that his country was effectively subsidising Canada by paying to import its products. Without this flow of capital, Canada “ceases to exist as a viable country”, he wrote.

Trump has previously spoken of using “economic force” to make Canada the 51st state of America. But he said he was not considering using military force – an assurance he has not given while stating his ambitions of taking the Panama Canal and Greenland.

The question of how to respond to Trump’s tariff threats and other statements about Canadian sovereignty has dominated the debate in Trudeau’s Liberal Party, with candidates vying to replace the PM after he announced his resignation in January.

On the other side of the Canadian political divide, Conservative leader Pierre Poilievre has also attacked Trump’s statements.

The King has found himself at the centre of a recent whirlwind of diplomatic meetings following Trump’s return to the presidency – some of which relate to the war in Ukraine.

On Sunday, he also met Ukrainian President Volodymyr Zelensky – who ended up in a row of his own with Trump at the White House on Friday.

Zelensky was in the UK to meet European leaders, who reiterated their support for Ukraine in its conflict with Russia – with American support apparently on the wane under Trump.