The Queen has been admitted to hospital with symptoms of gastroenteritis, Buckingham Palace said.
She had been due to visit Rome for two days this week, but the trip has been cancelled.
A Palace spokesman said: "The Queen is being assessed at the King Edward VII's Hospital, London, after experiencing symptoms of gastroenteritis.
"As a precaution, all official engagements for this week will regrettably be either postponed or cancelled."