The Queen will light the first of a chain of more than 1,000 beacons across Britain and the world to mark her 90th birthday, Buckingham Palace has announced.
On the day of her milestone anniversary - Thursday April 21 - the Queen will be joined by the Duke of Edinburgh, her eldest son the Prince of Wales and his wife the Duchess of Cornwall as she starts the principal beacon to celebrate her big day.
Buckingham Palace has revealed the Queen's diary of engagements for her birthday, which also includes a walkabout near Windsor Castle.
She will also unveil a plaque at the foot of Windsor's Castle Hill to mark The Queen's Walkway.
The 6.3km self-guided walking trail, designed by The Outdoor Trust, connects 63 points of significance in Windsor and celebrates the Queen's record as the country's longest reigning monarch.
She will then head out on a birthday walkabout - just like she did on her 80th - to see the expected crowds of well-wishers.
To coincide with the Queen's beacon-lighting ceremony, local authorities will host similar events across the country, making a network of beacons.
The beacons will be specially built gas-fuelled structures, traditional bonfires or braziers on top of a tall wooden post.
Members of the Army Cadet Force will be taking beacons to the top of the four highest peaks in the United Kingdom - Ben Nevis in Scotland, Mount Snowdon in Wales, Scafell Pike in England, and Slieve Donard in Northern Ireland.
There is a long tradition of celebrating royal jubilees, coronations and birthdays with the lighting of beacons. The last such royal celebration was for the Queen's Diamond Jubilee in 2012.
The Queen will also be out and about the day before her birthday, carrying out two engagements on April 20.
She will meet long-serving postmen and women during a visit to the Royal Mail Windsor delivery office to commemorate the 500th anniversary of the postal service.
Joined by Philip, the Queen will view the operation of the caller's office and see a demonstration of mail sorting, before watching a performance by the Royal Mail choir, made up of front-line staff from Bristol, who featured in the BBC series The Choir: Sing While You Work.
In her second engagement, the Queen will open a new bandstand in Alexandra Gardens, close to Windsor Castle, meeting school children who were involved in its decoration.
The bandstand was designed in recognition of the long-standing links between the Queen, the Armed Forces and Windsor, and to mark her record as Britain's longest reigning monarch.