The photograph shows Queen Elizabeth II wearing sapphire jewellery given to her by King George VI as a wedding gift in 1947. (Photo by David Bailey, photo courtesy of Buckingham Palace)

LONDON, Feb. 7 (Xinhua) -- Queen Elizabeth II reached another milestone on Monday when she became the only British monarch in history to celebrate a sapphire anniversary, marking 65 years on the throne.

The Queen spent the day at Sandringham in Norfolk, in line with her tradition of spending Accession Day, the anniversary of the day she became monarch, in private.

Since 2015, the Queen has been Britain's longest serving monarch, having then overtaken Queen Victoria's long reign.

Prime Minister Theresa May paid tribute to the Queen in a statement issued by 10 Downing Street: "It is a testament to her selfless devotion to the nation that she is not marking becoming the first monarch to reign for 65 years with any special celebration, but instead getting on with the job to which she has dedicated her life."

"She has truly been an inspiration to all of us and I am proud, on behalf of the nation, to offer our humble thanks and congratulations on celebrating her Sapphire Jubilee."

This year, Queen Elizabeth II will be 91 and is scaling down her activities by having her grandchildren Prince William and Prince Harry undertake more royal duties. But she has shown no signs of retiring as Britain's ruling monarch.