The Queen has been admitted to hospital as a precaution and will be assessed for symptoms of gastroenteritis.
Buckingham Palace said the 86-year-old monarch's engagements for the coming week, including a trip to Rome, have been cancelled or postponed.
The spokesman said Queen Elizabeth was in hospital for precautionary assessments and may remain there for a couple of days.
News of her illness emerged on Saturday after she was forced to cancel a trip to Swansea to mark St David's Day in a military ceremony.
The Queen is known for her robust health and celebrated her 60 years on the throne last year.
She was last hospitalised in 2003 when she had a knee operation, Reuters reports.