Today's visit by King Charles and Queen Camilla to Ottawa will ‘reinforce the power and the strength of the message’ to Donald Trump that ‘Canada is not for sale’.

That was the message of the country’s High Commissioner to the UK, Ralph Goodale, when they visited his Canada House headquarters in London last week.

Symbolically Charles and Camilla stood on a giant floor map of the country featuring historic points of interest including, ahem, The War of 1812 (fought by the US against Great Britain and its North American allies, which America vainly hoped would result in its conquest over British territory in Canada).

Not that you would have seen or heard anything other than a polite and interested smile from the King on the matter. As a constitutional monarch he is not permitted to wade in on such issues, of course.

And that has very much placed Buckingham Palace on a diplomatic high-wire in recent months, particularly given the British Government’s desire – no, need – for the King to pour oil on potentially choppy waters between Britain and the US given his genuinely ‘warm’ personal relationship with President Trump.

But, royal sources stress to me, Charles has been able to show his support in other, more subtle ways.

He has recently opened the doors of his home to two Canadian prime ministers, met a delegation from the Canadian senate and ceremonially handed over a new sword to Canada’s Black Rod.

In February (and this is a gesture very much emphasised by those in the know) he sent a Flag Day message to the people of Canada, which was repeated on Buckingham Palace’s social media channels.

King Charles talks to a Royal Canadian Mounted Police officer while visiting Canada House in London last week

US President Donald Trump with Canadian prime minister Mark Carney at the White House during a meeting at the start of this month

Canadians have rallied against the suggestion by President Trump that the country could become the US's 51st state

Put it this way, no such message was released last year.

While this may seem like small beef to some, the significance is not lost on diplomats either side of the Atlantic.

But there is no doubt that the King’s arrival in the Canadian capital today – his first as head of state – to undertake the State Opening of Parliament on Tuesday is ramping the messaging up by a decibel or two.

He will become only the second monarch – after Queen Elizabeth II – to attend the State Opening and deliver the speech setting out his Government’s legislative agenda.

His mother did so back in 1957 on her own first visit to the country as sovereign (happily providing a nice slice of historical symmetry).

In 1977, two decades later, she delivered just a ‘speech from the throne’, but otherwise left those regular official duties to her official in-country representative, the Governor-General.

So while there is limited precedence, the King’s decision to attend personally is clearly a rare honour – and one that is not, I can assure you, taking place by accident. ‘No-one should underestimate the significance of this,’ a well-placed source emphasises.

It certainly helps that the new prime minister Mark Carney, who enjoyed a cordial relationship with the King when he served as head of the Bank of England, was so quick out of the blocks to invite him.

King Charles and Queen Camilla during a visit to mark the 100th anniversary of Canada House last week. Charles is said to be looking forward to stepping on Canadian soil for the 20th time

The King enjoys a 'warm' personal relationship with Mr Trump, and the UK Government is looking to capitalise on that cordiality

But Canadians should also take heart that the King was equally speedy to accept, despite the wearisome political chest-beating and his own, acutely delicate role in such matters.

He is, I am assured, ‘hugely’ looking forward to stepping on Canadian soil once again (19 times previously as Prince of Wales).

And while his visit may seem short – involving little more than 24 hours in the country – he is packing as much into it as possible, including several ‘public moments’ giving him the ability to meet as many Canadians as possible.

Significantly, I can reveal, he is also considering another trip to the country in the ‘near to mid-future’. There is nothing in the diary yet, but it is something very much on the King’s mind.

‘Yes it’s challenging, less than 36 hours door to door, but although it feels very short and intense, that’s actually the best way to handle it,’ a well-placed source tells me.

‘The King and Queen will hit the ground running, cram in as much as they possibly can, all without even noticing the time difference. And the team will have him back in the UK for his regular scheduled [cancer] treatment as usual.

‘He had planned to go for a far lengthier visit last year but of course his diagnosis scuppered that, and then the Canadian electoral cycle.

‘But this [short visit] does not preclude the possibility of a longer royal tour of Canada in due course. There’s nothing in the diary yet, but it’s being looked at.’

And aides have every confidence the King will handle the constitutional challenges that await him with the dignity of the lifelong diplomat he is.

‘There is no other world leader like him, who is head of state of 14 countries. And yes, while it’s an interesting situation with Canada, the UK and the US, what he does have, not least because he is 76 years old and has been doing this for rather a long time, are warm, sincere personal relationships with political leaders across the globe and across the political spectrum. Setting aside inter-governmental issues, they all hold him in high personal regard,’ one royal source says.

I am told not to expect any ‘firecrackers’ in parliament tomorrow.

His speech will largely be written by the Canadian government – ‘they hold the pen’, another insider remarks – although Charles does plan to ‘top and tail’ with more personal reflections on Canada and his deep affection for the country he has known intimately since he was a boy.

‘Ultimately any monarch is at the service of his government – whether that be in the UK or Canada – and will do what is asked of them, and do it very well,’ one well-placed diplomatic source remarks.

Smiling, they add mischievously: ‘Perhaps having a King as a constitutional head of state may not seem, well, quite as radical to our American friends as it once was.’