Buckingham Palace says Queen Elizabeth's doctors are "concerned" about her health and "have recommended she remain under medical supervision."
The palace made the announcement in a statement on Thursday, saying the doctors made the recommendation after "further evaluation" that morning.
The statement also says the Queen "remains comfortable" at Balmoral Castle in Scotland. The estate is a summer retreat for the 96-year-old monarch.
The Queen's eldest son Prince Charles and his wife, Camilla, have reportedly arrived at the castle, and her grandson Prince William is also on his way.
The Queen appointed new Conservative Party leader Liz Truss as prime minister at Balmoral on Tuesday. Truss is the 14th prime minister invited by the Queen to form a government.
The BBC reported that it was the first time the monarch made such an appointment outside Buckingham Palace. Analysts say officials made the arrangement in consideration of the Queen's health.
The Queen did not attend some of the Platinum Jubilee events in June to celebrate her 70 years on the throne.