Gun salutes have rung out across the capital to commemorate the Queen making history by becoming the first British monarch to reach their Sapphire Jubilee.
It is 65 years to the day since the Queen acceded to the throne following the death of her father, George VI, on February 6 1952.
The Queen has spent the day privately at her Sandringham Estate in Norfolk, where the monarch spends her winter break.
But in London a 41-gun salute was fired by the King's Troop Royal Horse Artillery in Green Park at noon.
The Band of the Royal Artillery played a selection of celebratory music close to the firing position, and 89 horses pulled the six First World War-era 13-pounder field guns into position in the park.
The Honourable Artillery Company fired volleys at the Tower of London to mark the Sapphire Jubilee of the Queen's accession.
They fired a 62-round Royal Salute from Gun Wharf at 1pm - with an extra 21 volleys for the citizens of the City of London to show their loyalty to the monarch.
The Queen was in good spirits on Sunday as she greeted well-wishers after attending a church service at St Peter & St Paul in West Newton, Norfolk.
Three-year-old Jessica Atfield, who dressed specially for the occasion in a Union flag skirt, was delighted when she handed a bunch of flowers to the Queen.
The Queen, who missed church over Christmas due to a heavy cold, will undoubtedly be matter-of-fact about the historic occasion.
In 2015, when she thanked the nation for the kind messages after overtaking Queen Victoria to become the longest-reigning monarch in British history, she admitted bluntly that the royal record was ''not one to which I have ever aspired''.
She added: ''Inevitably, a long life can pass by many milestones. My own is no exception.''
The Queen celebrated her 90th birthday last year and had a busy schedule commemorating the occasion, with a walkabout, beacon-lighting and a black tie dinner for family and friends in Windsor on her actual birthday.
A weekend of national celebrations, including a party in The Mall, was held for her official birthday in June.
It is likely that any large-scale jubilee celebrations will be reserved for the Platinum Jubilee in 2022 - although any events will take into account the fact that Queen is due to turn 96 that year.
The Duke of Cambridge is about to become a full-time royal this summer and will increase his official duties on behalf of the Queen.
William and the Duchess of Cambridge were carrying out official engagements on Monday, but not to do with the Queen's record-breaking reign.
As the anniversary of Elizabeth II's accession to the throne marks the day her father died it is likely to be a reflective one.
She often spends Accession Day privately at Sandringham, staying there through the Christmas period. She usually makes her return to Buckingham Palace a few days later.
Theresa May paid tribute to the Queen in the House of Commons.
She said: "I am sure that the whole House will want to join me in sending our congratulations to Her Majesty the Queen as she marks her Sapphire Jubilee.
"It is testament to her majesty's selfless devotion to the nation that she is not marking becoming our first monarch to reign for 65 years with any special celebration, but instead by getting on with the job to which she has dedicated her life.
"And on behalf of the whole country I am proud to offer Her Majesty our humble thanks for a lifetime of extraordinary service, long may she continue to reign over us all."
Her comments were cheered by fellow MPs, and lifelong republican Jeremy Corbyn also thanked the Queen for her service.
The Labour leader told the Commons he echoed the PM's sentiments and that he wanted to wish the Queen well "at this auspicious time in her life and thank her for her service".
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