Elizabeth, 86, will have to cancel a visit to Rome and other engagements as she recovers, and experts said she may have to be rehydrated intravenously.
Buckingham Palace said the queen had experienced symptoms of gastroenteritis and was being examined at King Edward VII Hospital in London. It was the first time in a decade that she has been hospitalized.
"As a precaution, all official engagements for this week will regrettably be either postponed or canceled," the palace said. Her two-day trip to Rome was to start Wednesday. A spokeswoman said the trip may be "reinstated" at a later date.
The symptoms of gastroenteritis -- vomiting and diarrhea -- usually pass after one or two days, but they can be more severe in older or otherwise vulnerable people. Dehydration is a common complication.
The illness was first announced Friday, and Elizabeth canceled a visit Swansea, Wales, on Saturday to present leeks, a national symbol, to soldiers of the Royal Welsh Regiment in honor of St. David's Day. She instead spent the day trying to recover at Windsor Castle, but apparently had trouble kicking the bug.
A doctor not involved in the queen's treatment said that if medical officials determined that she is losing too much fluid, she would be rehydrated intravenously.
"Not everyone can keep up with oral hydration so it is pretty routine to go to hospital and have a drip and wait for the thing to pass and keep yourself hydrated," said Dr. Christopher Hawkey of the University of Nottingham's faculty of medicine and health sciences.
The National Health Service says the two most common causes of gastroenteritis in adults are food poisoning and the norovirus, a vomiting bug that typically afflicts between 600,000 and 1 million Britons each winter. British health guidelines advise that people with the norovirus avoid work for at least two days.
"It's very infectious and strikes in winter because people are indoors and it spreads more easily," Hawkey said.
Elizabeth has ruled since 1952 and is Britain's second longest-serving monarch, topped only by Queen Victoria in years on the throne.
Her husband, Prince Philip, 91, has had several hospital stays, but Elizabeth has rarely let sickness get in the way of her still-busy schedule.