The Queen is in hospital “as a precaution” where she is being assessed for symptoms of gastroenteritis, 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 Queen will remain in hospital under observation for around two days.
The Queen was last in hospital 10 years ago, our correspondent adds.
“The Queen is being assessed at the King Edward VII’s Hospital in London, after experiencing symptoms of gastroenteritis,” a palace spokesman said.
“As a precaution, all official engagements for this week will regrettably be either postponed or cancelled.”
Britain’s head of state is known for her robust health and devotion to duty, and rarely misses engagements.
The sovereign first felt unwell on Friday and cancelled a visit Saturday to a military celebration in the city of Swansea to mark St. David’s Day, the national day of Wales.
Instead of that engagement, she had been spending the weekend resting at Windsor Castle, west of London.
That she was taken to the royals’ regular hospital in central London suggests that her condition was not serious enough to warrant going to a hospital close to Windsor.
Queen Elizabeth and her husband Prince Philip were due to visit Rome on Wednesday and Thursday, when they were to meet Italian President Giorgio Napolitano.
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 coronation in 1953.