The Queen is in hospital as a precaution, while she is assessed for symptoms of gastroenteritis, Buckingham Palace says.
The 86-year-old monarch has been taken to King Edward VII Hospital in London, a palace spokesman said.
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 Windsor Castle, where she has been resting over the weekend.
A spokesman for the Queen said she was in "good health", besides the symptoms of gastroenteritis.
He said: "This is a precautionary measure.
"She was not taken into hospital immediately after feeling the symptoms. This is simply to enable doctors to better assess her."
The BBC's royal correspondent Peter Hunt says the Queen will remain in hospital under observation for about two days.
She was last in hospital 10 years ago for a minor knee operation.
The BBC's Ben Ando, outside the central London hospital, says there is a small police presence and that members of the press from around the world have gathered near the entrance.
There is no sign of any visitors to the Queen as yet, says our correspondent.
Downing Street said Prime Minister David Cameron had "passed on his best wishes" to her.
News of her illness emerged on Friday night, and she was forced to cancel a trip to Swansea on Saturday 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.
According to the BBC's royal correspondent Nicholas Witchell, the Queen may well have gone to hospital slightly unwillingly, as her inclination is not to make a fuss.
During last year's celebrations for the Diamond Jubilee, the Queen spent a rain-drenched day journeying down the Thames as part of the river pageant - after which her husband, the 91-year-old Duke of Edinburgh, was taken to hospital with a bladder infection.