Queen Elizabeth II was on Sunday admitted to a hospital in London after experiencing symptoms of gastroenteritis, said Buckingham Palace officials.
The 86-year-old monarch has been taken to King Edward VII Hospital in London, a palace spokesman said.
The spokesman for the Queen said that she was in good spirits and was otherwise in good health.
"This is a precautionary measure. She was not taken to the hospital immediately after feeling the symptoms. This is simply to enable doctors to better assess her," he said.
"The Queen is being assessed at the King Edward VII Hospital, London, after experiencing symptoms of gastroenteritis," the Palace said in a statement.
"As a precaution, all official engagements for this week will regrettably be either postponed or cancelled," the statement said.
Buckingham Palace said the monarch, who had taken ill on Friday, had been driven to the King Edward VII hospital in central London by private car.
She was driven to the hospital in the private car from Windsor Castle, where she had been resting.
The Queen, who celebrates her 87th birthday in April, is expected to remain in hospital for about two days.
The Queen and the Duke of Edinburgh were due to visit Italy on Wednesday for a two-day visit at the invitation of Italy's President Giorgio Napolitano.
The couple was due to attend a private lunch and also visit the Pantheon, the ancient Roman monument where two Italian kings are buried.
The Queen had on Saturday cancelled a planned visit to Wales. But she had 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.
Gastroenteritis infection can be transmitted through contact with an infected person or contaminated food and drink. Symptoms can include vomiting, fever and stomach ache.
The King Edward VII hospital, which is the preferred choice of hospital for the royal family, was at the centre of a media storm in December when the Duchess of Cambridge was admitted there after suffering from a severe form of morning sickness.
An inquiry was launched after two Australian radio DJs made a hoax call, pretending to be the Queen and Prince Charles, and asked for details of the duchess's medical condition.
Indian-origin nurse Jacintha Saldanha, who was at the receiving end of the hoax, was later found dead in a case of suspected suicide. The full inquest into her death is expected to reopen later in March.