The King and Queen said they are "greatly shocked and saddened" in a poignant statement following a fire at a Canadian military base over the weekend.
In a joint message, Charles, 77, and Camilla, 79, sent their "deepest sympathy and heartfelt condolences" following the "tragic incident" at Canadian Forces Base (CFB) Wainwright on Saturday.
The fire at the military base killed four members of the armed forces, with many more injured.
The royal couple wrote: "Our deepest sympathy and heartfelt condolences are with the families and loved ones of those who have lost their lives, and with all those who have been injured or otherwise affected at this most difficult time.
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"We extend our particular admiration and gratitude to the military and civilian emergency services, healthcare workers and all those who have responded with such courage and dedication in the most challenging circumstances.
Ending the message, "Charles R", the monarch, who is Head of State in Canada, said: "Above all, we send our special thoughts and prayers to the people of Alberta, and to the Canadian Armed Forces and their families."
What happened in the fire in Alberta, Canada?
The King's message came after four people were killed following a "fast-moving" fire at a military base in Canada, while dozens more are believed to be seriously injured.
The blaze broke out at 4 pm on Saturday at CFB Wainwright, one of the country's busiest military bases, and was spread quickly by strong winds in the area.
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It occurred during an exercise where soldiers had been practicing simulated attacks on an enemy.
Addressing the nation on Sunday, Canadian Prime Minister Mark Carney called the incident "one of the worst tragedies for the forces on Canadian soil in peacetime".
"The nation is in mourning for those brave members of our armed forces who we have lost," he said.
The four service members have been identified as Pte. Aidan Goulding, Pte. Shaun Farrell, Pte. Hayden Lykkemark, Sgt. Morgan Wong. Their next of kin have been notified.
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Of the 39 people who were taken to hospital after the fire in Alberta on Saturday, 19 are in a critical condition.
The Wainwright base is one of the country’s busiest military training bases, where soldiers prepare for combat operations and train with personnel from across the country.
Rescue teams have contained the fire and remain at the scene to prevent any flare-ups.
King Charles's close bond with Canada
King Charles is Head of State and Sovereign of Canada.
He was formally proclaimed King at Rideau Hall on 10 September 2022, following the passing of his mother, the late Queen Elizabeth II.
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In his declaration to the Accession Council at the time, the King said: "I am deeply aware of this great inheritance and of the duties and heavy responsibilities of sovereignty which have now passed to me. In taking up these responsibilities, I shall strive to follow the inspiring example I have been set in upholding constitutional government and to seek the peace, harmony and prosperity of […] the Commonwealth realms and territories throughout the world."
He travelled to Ottawa for a two-day visit in May 2025, which marked his 20th visit to the country over 50 years.
During the visit, Charles personally delivered a speech to open Canada's 45th Parliament – the first time a reigning King has opened a Canadian parliamentary session.
This week, the King and Queen will join Prince William for a rare joint outing to Wales, to commemorate the 60th anniversary of the Aberfan disaster.