Britain's 86-year-old Queen Elizabeth II was admitted to hospital for the first time in 10 years on Sunday as a precautionary measure, after struggling to shake off a bout of gastroenteritis.
A Buckingham Palace spokesman said the monarch's planned visit to Rome this week -- her first trip overseas since October 2011 -- had been called off.
Known for her robust health and devotion to duty, Britain's head of state rarely misses engagements.
"The Queen is being assessed at King Edward VII's Hospital in London after experiencing symptoms of gastroenteritis. As a precaution, all official engagements this week will be either postponed or cancelled," the palace spokesman said.
He said she was in "good spirits" and was otherwise in "good health".
"This is a precautionary measure,"the spokesman added.
"She was not taken into hospital immediately after feeling the symptoms.
"This is simply to enable doctors to better assess her."
Broadcasters reported the queen would remain in hospital under observation for about two days.
After coming down Friday with gastroenteritis -- an infection of the stomach and bowel with symptoms commonly including vomiting, diarrhoea and dehydration -- the sovereign missed celebrations in Swansea on Saturday to mark St. David's Day, the national day of Wales.
Instead of that engagement, she had been resting at Windsor Castle, west of London, where she and her husband Prince Philip usually spend the weekend.
That she was driven to the royals' regular hospital in central London suggests her condition did not warrant finding a hospital close to Windsor.
In December, her grandson Prince William's pregnant wife Catherine spent three days at the private hospital with acute morning sickness.
A nurse at the hospital was found hanged after taking a hoax call from two Australian radio presenters posing as the queen and William's father Prince Charles, which led to details of Catherine's condition being made public.
Police officers were stationed outside the hospital entrance on Sunday.
Queen Elizabeth was due to visit the Royal Navy frigate HMS Lancaster in the east London docklands on Tuesday.
The sovereign and Prince Philip were then supposed to visit Rome on Wednesday and Thursday, when they were to meet Italian President Giorgio Napolitano.
It is not yet known if the Rome trip will be rescheduled.
Queen Elizabeth was last admitted to hospital 10 years ago.
She had an operation at King Edward VII's Hospital to remove cartilage from her left knee on December 12, 2003 and had some benign facial lesions removed at the same time.
She was last forced to pull out of an investiture ceremony -- where she presents honours including knighthoods -- in October last year due to a bad back.
The monarch celebrated her diamond jubilee in 2012, marking her 60 years on the throne. This year further ceremonies will be held to mark the six decades since her 1953 coronation.