All the monarch’s engagements for the coming week, including a trip to Italy, have been canceled or postponed as a precaution, the spokesman said.
The queen had been due to visit Italy as a guest of President Giorgio Napolitano on March 6-7.
At 86, the queen is known for her robust health. She was last hospitalized in 2003 when she had a knee operation.
The spokesman said the queen was at the King Edward VII Hospital for precautionary assessments and may remain there for a couple of days.
She was last at the hospital last year, visiting her husband Prince Philip who was treated for a bladder infection.
Queen Elizabeth, who celebrated 60 years on the throne last year, usually maintains a busy schedule of public and private engagements although in recent years she has cut down on her once hectic timetable of foreign trips.
The queen has enjoyed a period of high popularity over the past two years.
The wedding of her grandson Prince William to Kate Middleton in 2011 generated significant goodwill towards the royal family. The couple are now expecting a child amid huge media interest in Britain.
The queen’s own Diamond Jubilee celebrations cemented her popularity, as did her participation in the opening ceremony of the Olympic Games, when she appeared alongside the fictional spy James Bond in a video sequence that was one of the highlights of the show.