She was expected to stay for a couple days but spent just 24 hours at King Edward VII's Hospital after being admitted on yesterday afternoon.
The British monarch, dressed in a red coat, appeared relaxed and smiled as she walked to her waiting Bentley outside the hospital in central London. She is now likely to spend time convalescing at Windsor Castle.
Her next public outing is scheduled to be next Monday, when the Queen will carry out a day of Commonwealth-related engagements, including a service at Westminster Abbey where she will address a 2,000-strong congregation.
She did not wish to "inconvenience" staff at the hospital by having to make arrangements for royal visitors and wanted to keep her hospital stay "as low key as possible".
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Buckingham Palace had said the Queen had been admitted as a precaution and was otherwise in "good health and good spirits".
All official engagements for this week, including the Queen's trip to Rome, have either been cancelled or postponed.
According to royal sources, the Prince of Wales and the Duchess of Cornwall, who spent the weekend in Wales, and the Duke and Duchess of Cambridge and Prince Harry, who were in Switzerland at the weekend for a friend's wedding, are being kept updated on her health.