Queen Elizabeth II, the world's longest-reigning living monarch, has celebrated her Sapphire Jubilee as Britain commemorates 65 years since she ascended the throne.

The 90-year-old monarch, who became the kingdom's longest-reigning sovereign in 2015, did not publicly mark the occasion herself, but a 41-gun royal salute was fired in a central London park to honour the landmark.

"Today's Sapphire Jubilee marks yet another remarkable milestone for our remarkable Queen," Prime Minister Theresa May said in a statement.

"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."

Queen Elizabeth II wears sapphire jewellery given to her by her father King George VI. ( AP: David Bailey )

Elizabeth became Queen aged 25 on February 6, 1952, following the death of her father George VI.

She is the 41st monarch in a royal line that traces its origin back to Norman King William the Conqueror who claimed the throne in 1066.

When she overtook her great-great-grandmother Queen Victoria's record of 63 years on the throne, she remarked it was not something to which she had ever aspired.

Buckingham Palace said she would spend Monday at her residence in Sandringham, eastern England, as was usual.

However, the Queen's office released a 2014 portrait showing her wearing a suite of sapphire jewellery she received from her father as a wedding gift in 1947.

She turned 90 on April 21, 2016, and has been the world's oldest monarch since the death of Saudi Arabia's King Abdullah in 2015.

The Queen has cut back on international tours but still regularly performs official duties around Britain and remains hugely popular with Britons, with an approval rating of about 80 per cent.

In December she said she would reduce the number of her patronages, passing on her role at dozens of charities, academic institutions and sporting groups to other members of the royal family.

She has four children, eight grandchildren and five great-grandchildren.

Her eldest son, Prince Charles, is next in line to the British throne.

Sorry, this video has expired The King's Troop Royal Horse Artillery stage a 41-Gun Royal Salute to celebrate Queen Elizabeth II's Sapphire Jubilee. (Photo: AP/Matt Dunham)

Reuters/AP