The 86-year-old British monarch does not want to "inconvenience" staff at the King Edward VII Hospital in London by having to make arrangements for royal visitors, insisting her hospital stay should be "as low key as possible", according to royal sources.
This is the first time in 10 years that she has been hospitalised and Buckingham Palace said she could be back home as early as tomorrow.
All official engagements for this week, including the Queen's trip to Rome, will be either cancelled or postponed.
News of her illness first emerged on Friday night when she cancelled a trip to Swansea on Saturday to mark St David's Day in a military ceremony. She was taken to the hospital yesterday from Windsor Castle, using a private car rather than an ambulance, and walked to and from the car without help.
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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.
Her treatment, which has not been disclosed, is expected to include rehydration and tests to establish if the illness has been caused by an infection or an underlying problem.
A reception at Buckingham Palace tomorrow for MPs and MEPs will go ahead with other members of the royal family present.