The Queen has cancelled all of her engagements for next week after being admitted to hospital.
Buckingham Palace said she was experiencing symptoms of gastroenteritis on Sunday.
All official engagements for this week will be either postponed or cancelled as a precaution, the Palace added.
She has been admitted to King Edward VII Hospital in central London, where Kate Middleton recently received treatment for morning sickness.
A spokesman for the Queen said she was in "good spirits" and was otherwise 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 Queen's sickness was first announced on Friday, and she spent Saturday resting at Windsor.
She has undertaken a number of engagements during the last week. On Tuesday she met the Archbishop of Canterbury at Buckingham Palace, and on Thursday she presented a host of Olympic stars with honours during an investiture ceremony.
Gastroenteritis is an infection of the stomach and bowel, and the most common symptoms are vomiting and repeated episodes of diarrhoea.
The Queen presented a host of Olympic stars with honours during an investiture ceremony at Buckingham Palace on Thursday.
She and the Duke of Edinburgh were due to fly out to Rome for a two-day visit next week.
A spokesman for the Queen said she was in "good spirits" and was otherwise 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."
Downing Street said that Prime Minister David Cameron had passed on his best wishes to the Queen.