The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace says.
"Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," they said in a statement.
"The Queen remains comfortable and at Balmoral."
Her immediate family has been informed.
The announcement comes after the Queen, 96, pulled out of a virtual Privy Council on Wednesday, with doctors advising her to rest.