The queen was taken to the hospital "as a precautionary measure," the spokesman said, and is expected to stay at London's King Edward VII's Hospital for two days.
Gastroenteritis is an inflammation of the lining of the stomach and intestines. It is commonly known as "stomach flu," but is not caused by the influenza virus, according to the U.S. Centers for Disease Control and Prevention.
"A decision was taken on medical advice to take her from Windsor (Castle) to hospital," the spokesman said.
"She is otherwise in good spirits and good health," the spokesman said.
All official engagements this week will be postponed or canceled, the palace said.
"We wish the Queen well and wish her a speedy recovery," Prime Minister David Cameron's office said.
The 86-year-old queen took the throne in 1952. She already had canceled a planned trip to Wales on Saturday after exhibiting symptoms of the illness, Buckingham Palace said.
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