The Queen has been admitted to hospital with symptoms of gastroenteritis.
A spokesman for the Queen said she was in "good spirits" and was otherwise in "good health".
He 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."
The Queen had been due to visit Rome for two days this week, but the trip has been cancelled.
A Buckingham 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 Queen's sickness was first announced on Friday, and she spent yesterday resting at Windsor.
She was due to attend a military celebration in Wales during the country's St David's Day celebrations, but was forced to cancel the plans.
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.
Downing Street said that Prime Minister David Cameron had passed on his best wishes to the Queen.
