Queen Elizabeth went into hospital in London on Sunday with symptoms of gastroenteritis, a Buckingham Palace spokesman said.
The 86-year-old monarch first developed symptoms of the stomach bug on Friday and the decision to take her to hospital was a precautionary measure. She was in good spirits and her condition was not deteriorating, the palace spokesman said.
All the queen’s engagements for the coming week, including a trip to Italy scheduled for March 6-7, have been cancelled or postponed. It is unusual for the monarch, who is very diligent in carrying out her royal duties, to cancel any engagement.
The queen last appeared in public on Thursday, when she bestowed honours on British Olympic medallists including heptathlon star Jessica Ennis. The queen appeared well and happy in a salmon pink outfit in photographs of that event.
The head of state, who last year celebrated 60 years on the throne, is known for her robust health. She was last hospitalised in 2003 when she had a knee operation.
The palace spokesman said she had been spending the weekend at Windsor Castle, outside of London, and was driven by private car from there to the King Edward VII hospital in central London at about 3 p.m. (1500 GMT).
The spokesman said she was there for precautionary assessments and may remain there for a couple of days.
She was last at the hospital last year, visiting her husband Prince Philip when he was treated there for a bladder infection.
Queen Elizabeth usually maintains a busy schedule of public and private engagements although in recent years she has cut down on her once hectic timetable of foreign trips.
The queen has enjoyed a period of high popularity over the past two years.
The wedding of her grandson Prince William to Kate Middleton in 2011 generated significant goodwill towards the royal family. The couple, now known as the Duke and Duchess of Cambridge, are expecting a child amid huge media interest in Britain.
The queen’s own Diamond Jubilee celebrations last year cemented her popularity, as did her participation in the opening ceremony of the London Olympic Games, when she appeared alongside the fictional spy James Bond in a video sequence that was one of the highlights of the show.
The private King Edward VII is the hospital of choice for members of the royal family. The Duchess of Cambridge was treated there for acute morning sickness in December.
The hospital found itself at the centre of a global media storm after a crank caller from an Australian radio station, posing as the queen, obtained details of the duchess’s condition. The nurse who took the call later committed suicide.