British Queen Elizabeth II was admitted to hospital on Sunday due to symptoms of gastroenteritis, the Buckingham Palace said.
The palace described her as "in good spirits," while the move was a "precautionary measure."
"She was not taken into hospital immediately after feeling the symptoms. This is simply to enable doctors to better assess her," a spokesman with the palace said.
The queen is now in the King Edward VII hospital, where Kate Middleton stayed after she was pregnant.
A scheduled visit to Italy was cancelled due to the queen’s health reasons.
Elizabeth II, who will be 87 next month, has always been in good health. This is the first time in recent 10 years she is taken to hospital, with the last time in 2003 when she had a knee surgery.