Along with the king's message, the royal family also shared a previously unreleased photo of Elizabeth that was captured at Buckingham Palace in 1968, when the queen was 42, some 16 years into her reign.
Following the death of his long-serving mother at the age of 96, Charles ascended the throne on Sept. 8, 2022.
"Today, on the solemn anniversary of the passing of Her Late Majesty Queen Elizabeth II, our thoughts are with His Majesty King Charles III and the whole Royal Family," said Prime Minister Rishi Sunak.
In a statement, Sunak said that since the queen's passing, the nation's "gratitude for such an extraordinary life of duty and dedication, only continues to grow."
"While we continue to mourn Her Late Majesty's passing, we should be proud that this remarkable legacy of service-and this remarkable bond-continues to grow today under the reign of His Majesty The King," he added.
John McFall, speaker of the House of Lords, marked the anniversary on behalf of the house.
"Today's anniversary of the loss of Her Majesty Queen Elizabeth II is a moment to reflect on her unparalleled legacy of service to the United Kingdom and Commonwealth," he said in a statement.
Lindsay Hoyle, speaker of the House of Commons, also commemorated Elizabeth II, saying: "We reflect on her life, legacy and dedication to the UK, the British overseas territories and the Commonwealth."
"Today, we reflect on her legacy and embrace the late Queen's spirit of public service as our guide towards a better future," Keir Starmer, head of the main opposition Labour Party, said on X, formerly known as Twitter.
