Ever since she turned 65 in 1991, speculation has been rife about whether she would step aside for her eldest son, Prince Charles.
But her aides and royal commentators says there is still no prospect of he sprightly ruler giving up the top job.
“The Queen has a phenomenal drive and energy and I think the fact that she still works is what keeps her going,” says Ingrid Seward, editor-in-chief of Majesty magazine.
“The Queen doesn’t want to put her feet up. This is what she wants to do, and this is what she’s going to do for as long as she’s fit and able to do so.”
Her birthday comes months after she surpassed the 23,226 days her great-great-grandmother Queen Victoria spent on the throne to become Britain’s longest-reigning monarch.
Born on April 21, 1926 in central London, Elizabeth still hosts state visits, presides over the annual state opening of parliament and holds a weekly audience with the prime minister.
While she has cut back on her once demanding schedule of foreign trips, those close to her say only incapacity would stop her from performing her royal duties. Last year she carried out 341 official engagements.
Although Elizabeth’s father died at the relatively young age of 56, her mother, who was known as Queen Elizabeth the Queen Mother, lived until 101 and was still appearing in public almost up until her death in 2002.
“As she turns 90, she’s a remarkably energetic and guiding force for her family,” her grandson Prince William said in a speech during his India tour. “She maybe my grandmother, but she’s also very much the boss.”
The Queen’s week of celebrations kicks off with a visit to the Royal Mail Windsor delivery office on April 20 to mark the 500th anniversary of the postal service, followed by the opening of a new bandstand at Alexandra Gardens in Windsor.
On her official birthday, April 21, the Queen unveils a plaque marking the start of the four-mile Queen’s Walkway at the foot of Castle Hill in Windsor.
The Queen and the Duke of Edinburgh will then go on a walkabout to greet members of the public. Later that day she’ll also light the first in a chain of 1000 beacons across England and worldwide that celebrate her birthday.
On April 22, the Queen hosts President Barack Obama for a private luncheon at Windsor Castle.
“”I’m glad to be the very first President of the United States to wish Her Majesty a happy 90th birthday,” says the President in an Instagram post.
“For nine decades Her Majesty has been a source of strength and inspiration, not only for the people of Britain, but for millions of people around the world.”