The United Kingdom's King Charles III is due to make a speech to open Canada's 45th parliament on Tuesday as part of his historic two-day visit to the country.

The trip is seen as sending a message of support amid United States President Donald Trump's threats of annexation.

The 76-year-old sovereign, who serves as Canada's head of state within the Commonwealth, is due to deliver the throne speech, a formal address that lays out the government's agenda. It is the first time a monarch has given this speech for almost 50 years.

Canada's Prime Minister Mark Carney, fresh from an election victory fueled by anti-Trump sentiment, invited the monarch and Queen Camilla earlier this month. When announcing the visit, Carney said "Canada has a steadfast defender in the sovereign".

"His Majesty King Charles III will read the speech from the throne and present the plan of our government, our priorities, as Canada faces a pivotal moment in its history, the biggest transformation of the global trading system since the fall of the Berlin Wall," Carney was quoted as saying by the Associated Press, or AP.

While the throne speech will present the government's objectives, the king must follow strict rules about its delivery, reported The Guardian newspaper.

Royal historian Justin Vovk told the paper that within a constitutional monarchy, the king operates "inside a box that is defined by parliament". He noted that this ensures no unelected person can affect policy and laws, meaning "any influence the monarch exerts is done through subtlety".

Some observers suggested that the king's mother, the late Queen Elizabeth II previously used subtle signals like wearing specific brooches to communicate political positions, and Charles has continued this diplomatic approach.

He recently wore Canadian military insignia on his British admiral's uniform and planted a maple tree at Buckingham Palace.

US Ambassador to Canada Pete Hoekstra downplayed the symbolic nature of the royal visit. "If there's a message in there, there's easier ways to send messages. Just give me a call. [Mark] Carney can call the president at any time," he told CBC News.

While Charles and Camilla's schedule includes community meetings and a street hockey ceremony, the visit has reignited debate about whether Canadians still want a monarch as their head of state.

AP reported that most Canadians remain indifferent to the monarchy, but said the relationship highlights key historical differences between Canada and the US.

The US had a revolution to gain independence from Britain, while Canada evolved peacefully into a constitutional monarchy.

"We are different and the king illustrates that," former Quebec provincial premier Jean Charest said. "If you look at why King Charles is reading the speech from the throne than you have to then acknowledge Canada's story."