The Queen will spend another day in hospital as she continues to battle the stomach virus gastroenteritis, reports say.
Her Majesty, 86, is under the care of doctors at King Edward VII Hospital in London, the same private hospital which treated Kate Middleton for acute morning sickness in December.
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The royal, in hospital for the first time in ten years, has cancelled all engagements including a planned trip to Rome with the Duke of Edinburgh.
On Monday, Prince Philip was due to visit the Royal Thames Yacht Club in Knightsbridge, which would have made a visit to his wife in the King Edward VII Hospital likely following the engagement.
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."
News of the Queen's illness came on Saturday when she was forced to cancel a visit to the Welsh city of Swansea where she was due to mark St David's Day.
As a result of her illness, the Queen spent the day recuperating at Windsor Castle before aides decided it was safer for her be in hospital.
The 86-year-old was well enough on Sunday to meet an employee privately and award her a medal for long service before travelling by private car to the King Edward VII hospital.
Sources close to the stoic monarch say the Queen will want minimal "fuss" around her short-lived sick leave. In 60 years as Britain's sovereign, Her Majesty has rarely cancelled engagements due to ill-health and the last time she was in hospital was in 2004 for a knee operation.