On Wednesday, Buckingham Palace said the Queen was in 'good spirits'.
But it is believed doctors ordered her to rest after a slew of work commitments since returning from Balmoral at the start of October.
Last week she was forced to start using a walking stick in public for the first time, a stark reminder of her advancing years.
And on Tuesday night she hosted a major summit for global investors and politicians at Windsor Castle, standing on her feet and shaking hands for around 40 minutes.
Royal sources said there was no cause for concern, and stressed the decision to cancel her trip was purely a 'precautionary' measure.
But it is highly unusual that any major engagement would be cancelled at the last minute, suggesting that staff are taking no chances with the elderly head of state's health.
The monarch, who will celebrate her Platinum Jubilee next year, stoically continues to meet hundreds of work commitments each year – 30 years after most people have retired.
She is largely in good health but does suffer from recurrent back and knee pain, and the odd seasonal cold.
After almost 70 years on the throne, she is the world's longest-reigning monarch.
But she has faced a tumultuous 18 months, including the death of husband Philip in April, as well as the acrimonious departure of Prince Harry and wife Meghan.
The Queen's five previous hospital visits:
JULY 1982 – WISDOM TOOTH
The Queen had a wisdom tooth extracted at King Edward VII's hospital.
JANUARY 1994 – BROKEN WRIST
She broke her left wrist in a horse riding accident, but the injury wasn't diagnosed for 24 hours, during which time she remounted her horse and rode back to Sandringham.
She was taken to hospital in King's Lynn where she had an X-ray and her wrist was set in a plaster cast.
JANUARY 2003 – KNEE SURGERY
The Queen underwent surgery on her right knee during two nights at the King Edward VII's hospital.
She injured the knee just before Christmas while walking on uneven ground during a private visit to Newmarket.
DECEMBER 2003 – KNEE SURGERY
The Queen was admitted to King Edward VII's for surgery to remove torn cartilage in her left knee.
She also had two small lesions removed from her face during the two-night stay. Palace officials said the lesions were benign.
The Queen left hospital with two stitches under her left eyebrow and a black mark under her eye and wore a grey trouser suit, hiding the bandaging on her knee.
MARCH 2013 – GASTROENTERITIS
She stayed overnight at King Edward VII's with symptoms of gastroenteritis after she struggled to recover from a stomach bug.
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