The Queen is beginning her 90th birthday celebrations a day early as she heads out and about in Windsor.
With a visit to meet long-serving postmen and women at a Royal Mail delivery office near her Berkshire castle and the opening of a new bandstand, the public celebrations of the monarch's personal anniversary will get under way.
The Queen turns 90 on Thursday - one of many milestones in her record breaking reign. She is already Britain's longest reigning monarch, but on her birthday on April 21, she becomes the country's first nonagenarian sovereign.
On her last day as an 89 year old, the Queen will commemorate the anniversary of another British institution - the postal service - by touring the Royal Mail Windsor delivery office.
500 years ago in 1516, Henry VIII knighted Brian Tuke, the first Master of the Posts - a move that was the catalyst for the creation of the Royal Mail we know today.
Joined by the Duke of Edinburgh, 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.
A series of 10 Royal Mail postage stamps marking the monarch's birthday have been unveiled to coincide with the visit - with Prince George featuring on a stamp for the first time.
The image of the Duke and Duchess of Cambridge's two-year-old son was captured by photographer Ranald Mackechnie as part of a specially commissioned wider portrait featuring four generations of the House of Windsor - the Queen and three future monarchs - the Prince of Wales, the Duke of Cambridge and George.
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 the nation's longest reigning monarch.
On her actual birthday on Thursday, the Queen will step out of her Windsor Castle home, where she has been staying during Easter Court, and go on a walkabout in the town centre, meeting the crowds who will gather to catch a glimpse of her on her big day.
The head of state - with Philip, the Prince of Wales and the Duchess of Cornwall at her side - will later light the first in a chain of more than 1,000 beacons across Britain and the world.
Heir to the throne Charles will stage a lavish private family dinner for the royal matriarch in the evening in the castle - with the event said to be being overseen by his trusted former valet Michael Fawcett.
In June, thousands of well-wishers will gather for celebrations planned to mark the Queen's official birthday.
A series of events will be staged, from a St Paul's Cathedral service of thanksgiving to the traditional Trooping the Colour ceremony, and a sit-down celebration for 10,000 people on The Mall called the Patron's Lunch.