The Queen is in "good spirits" after being admitted to hospital with symptoms of gastroenteritis, Buckingham Palace said.
She was struck with the stomach bug on Friday and was forced to pull out of engagements for the coming week, but the 86-year-old was "otherwise in good health", according to a spokesman.
After cancelling a visit to Wales on Saturday she chose to rest at Windsor, but the monarch was admitted to King Edward VII's Hospital in London on Sunday as a "precaution".
The Queen was well enough earlier in the day to meet privately with an employee to award her a medal for long service.
She is likely to stay at the hospital for two days, so she has been forced to cancel a visit to HMS Lancaster in London on Monday and a reception for MPs and MEPs at Buckingham Palace will now be hosted by another member of the royal family.
A two-day tour to Rome with the Duke of Edinburgh, which was due to begin on Thursday, has also been called off to allow the Queen to rest. A Palace spokesman said: "The Queen is being assessed at the King Edward VII's Hospital, London, after experiencing symptoms of gastroenteritis. As a precaution, all official engagements for this week will regrettably be either postponed or cancelled."
The spokesman added: "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."
It is believed the Queen will not be taking visitors, so as to cause minimal fuss.
Downing Street said that Prime Minister David Cameron had passed on his best wishes to the Queen. A handful of police officers stood guard at the entrance to the hospital, while a pack of television crews, photographers and reporters waited outside.
The Queen, who is 86, 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.