Queen Elizabeth is in hospital “as a precaution”, while she is assessed for symptoms of gastroenteritis, Buckingham Palace says.
The 86-year-old monarch has been taken to King Edward VII Hospital in London, a palace spokesman said.
All official engagements for this week, including the Queen’s trip to Rome, will be either cancelled or postponed.
The Queen was driven to hospital by private car on Sunday, and the palace said she was “in good spirits”.
She had earlier carried out a medal presentation at Windsor Palace, where she has been resting over the weekend.
The BBC’s royal correspondent Peter Hunt says the Queen will remain in hospital under observation for about two days.
The Queen was last in hospital 10 years ago for a minor knee operation.
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