The Queen is being treated in hospital "as a precaution" after suffering from the symptoms of gastroenteritis.
Her Majesty, who is aged 86, spent the night at the King Edward VII's Hospital in central London.
The monarch is in "good spirits" and is otherwise in good health, according to her spokesman.
She was driven to the hospital by private car at about 3pm yesterday and is expected to remain there for a couple of days.
The spokesman 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 was last in hospital in 2003 when she had a knee operation.
All official engagements planned for this week will be postponed or cancelled, Buckingham Palace said.
These include a visit to HMS Lancaster in London today, and a reception for MPs and MEPs at Buckingham Palace, which 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.
The Queen had been due to meet Italian President Giorgio Napolitano, who sent her his "best wishes for a speedy recovery".
Prime Minister David Cameron also wished the Queen well.
The Queen's sickness was first announced on Friday. She had been due to attend a military celebration in Swansea on Saturday for St David's Day celebrations, but was forced to cancel because of the illness.
Instead, the monarch spent the day resting at Windsor Castle.
Before her admission to hospital she presented a female member of staff an award for long service at Windsor.
Gastroenteritis is an infection of the stomach and bowel. The most common symptoms are vomiting and repeated episodes of diarrhoea.
The two most common causes of gastroenteritis in adults in the UK are norovirus and food poisoning.
Medical experts said she would probably be treated with a drip. It is important to ensure patients are given combinations of sugar and salt to help fluids be absorbed.
Her Majesty has rarely missed a royal engagement due to ill health in recent years.
She was last forced to pull out of an investiture ceremony in October last year after suffering with a bad back.
She also missed a visit to the British Museum in October 2011 because of a cold.
The Duchess of Cambridge was treated at King Edward VII's Hospital last December for acute morning sickness.