The Queen has been taken to hospital "as a precaution" after suffering symptoms of gastroenteritis.
Her Majesty, who is 86, is being assessed by doctors at the King Edward VII 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 around 3pm and is expected to remain at the medical facility 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."
All official engagements planned for this week will be postponed or cancelled, Buckingham Palace said.
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.
The Queen's sickness was first announced on Friday.