Queen in hospital with stomach bug, Buckingham Palace says
A palace spokesperson said the 86-year-old monarch had been taken to King Edward VII Hospital in London.
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 Buckingham Palace.
The BBC’s royal correspondent Peter Hunt says the Queen will remain in hospital under observation for around two days.
The Queen was last in hospital 10 years ago, our correspondent adds.
Rome trip off
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.
Gastroenteritis causes inflammation of the stomach lining and intestines.
The infection can be transmitted through contact with an infected person or contaminated food and drink. Symptoms can include vomiting, fever and stomach ache.
The Queen had been due to spend two days in Rome with the Duke of Edinburgh next weekend, at the invitation of Italy’s President Giorgio Napolitano.
It is not now clear whether the visit will be re-scheduled.
A reception at Buckingham Palace on Tuesday for MPs and MEPs will go ahead with other members of the royal family present.