The Queen of England 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 Castle, where she has been resting over the weekend.
A spokesman for the Queen said she was in “good health”, besides the symptoms of gastroenteritis.
He said: “This is a precautionary measure.
“She was not taken into hospital immediately after feeling the symptoms. This is simply to enable doctors to better assess her.”
According to media correspondents, 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.
Currently, outside the central London hospital, there is a small police presence.
There is no sign of any visitors to the Queen as yet.