"The Queen has left The King Edward VII's hospital, having been admitted briefly as part of the assessment of symptoms of gastroenteristis," Buckingham Palace said early afternoon Monday, London time.
Royal sources had said Her Majesty, 86, seemed in "good spirits" Sunday, and that the admission to the hospital was strictly precautionary.
Earlier Sunday, she had even given an award for longstanding service to a member of staff, but is thought to have taken a turn for the worse after lunch.
Said her spokespeople at the Palace. "As a precaution, all official engagements for this week will regrettably be either postponed or canceled."
With the cancellation, her scheduled trip to Rome, where she was going to mix a private visit with an official date, has been put off.
Still, Italy's president Giorgio Napolitano – who had specially invited the Queen to visit his capital – was one of the first to send his good wishes for her recovery. British Prime Minster David Cameron also sent a message.