CLICK ON PHOTO TO ENLARGE
But a speedy recovery from gastroenteritis meant she was able to return to Buckingham Palace less than 24 hours later.
It was the Queen's first hospital stay in ten years, and all her official engagements, including a visit to Rome, were cancelled or postponed.
She was driven by private car from Windsor to the hospital on Sunday on the advice of her physician, Professor John Cunningham. A spokesman for the Queen insisted the monarch was in "good spirits" and that her stay in hospital was "a precautionary measure".
He added: "She was not taken into hospital immediately after feeling the symptoms. This is simply to enable doctors to better assess her".
King Edward VII is the same hospital that treated the Duchess of Cambridge last year when she developed severe morning sickness.
Watch: Prince William takes George to meet his sister
It's a little princess!