Queen Elizabeth has been taken to hospital after falling ill with the stomach infection gastroenteritis.
A Buckingham Palace spokesman said that the 86-year-old monarch is being treated at the King Edward VII hospital in London "as a precaution".
The hospital is the same one where Kate Middleton, Duchess of Cambridge, received treatment for morning sickness late last year.
All of the Queen's official engagements this week, including a two-day visit to Rome, have been cancelled or postponed.
After becoming ill on Friday, she had to cancel her attendance at a St David's Day military parade in Swansea, Wales.
Gastroenteritis is a stomach illness often caught through food poisoning or the norovirus. Symptoms include vomiting, diarrhoea and dehydration.
The Queen is expected to remain in hospital under observation for a few days. A palace spokesman said she was "in good spirits" despite her illness.