It was the first time the new king has signed a letter of condolence since the end of a period of national mourning on Tuesday.
The message was sent to Canada's Governor General, Mary Simon, after Storm Fiona hit the country's coastline. Officials said on Monday that at least one person died after being washed out to sea in the eastern province of Newfoundland.
“My wife [Camilla] and I were most concerned to hear of the appalling devastation caused by storm Fiona and particularly wanted to send our profound sympathy to the people of Atlantic Canada whose lives, livelihoods and properties have been so badly affected by this disaster,” King Charles' letter read.
“We have fond memories of our recent visit to your beautiful region and know that your resilience and sense of community will help you through these unbelievably difficult times.
“We would also like to express our deep appreciation to the first responders, the military and to community members, who are doing so much to support others during this extremely challenging period.
“Our thoughts and prayers are very much with all of you as you work to recover and rebuild.”
Other letters have started rolling off the Buckingham Palace franking machine using the new King Charles III cipher — his official monogram.
It is understood the letters are messages of appreciation for the cards and letters of condolence received by the royal family following Queen Elizabeth II’s death.
They the letters will be posted later this week.
David White, garter king of arms, and senior herald at the College of Arms which produced the image of the new king’s monogram, said the cipher would become a familiar image to the public.
“It’s the very personal mark of the sovereign and whereas the royal arms don’t necessarily change from reign to reign, the cipher does,” he said.
“I think it will become very familiar, I imagine most people are familiar with the previous cipher — the late Queen’s EIIR — without really realising it.”
Mr White was the official who read the proclamation declaring the new king from a balcony at St James’s Palace, following the queen’s death on September 8.
Tim Noad, heraldic artist and calligrapher at the College of Arms in London, created 10 designs that were put before the king, who chose his preferred monogram.
He also designed the queen’s golden, diamond and platinum jubilee medals and created the elaborate and intricately written “instrument of consent” from the queen. This formally approved the marriage of William and Kate, now the Prince and Princess of Wales, in 2011.
The cipher features the king’s initial C intertwined with the letter R for Rex — Latin for king. The III within the R denotes Charles III, with the crown above the letters.
The monogram is Charles’s personal property and a Scottish version features the Scottish crown.
It will appear on government buildings, state documents and on some post boxes in the coming months and years. The decision to replace ciphers is at the discretion of individual organisations.