The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has said.
"Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision," a statement said.
"The Queen remains comfortable and at Balmoral."
Prince Charles has travelled to Balmoral.
The Duchess of Cornwall has also travelled there and the Duke of Cambridge is on his way.
The announcement comes after the Queen, 96, pulled out of a virtual Privy Council on Wednesday, with doctors advising her to rest.
Prime Minister Liz Truss said the "whole country" would be "deeply concerned" by the news.
"My thoughts - and the thoughts of people across our United Kingdom - are with Her Majesty The Queen and her family at this time," she added.
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The Queen appointed Ms Truss as prime minister at Balmoral on Tuesday, instead of travelling to London for the event.
During her 70-year reign the Queen has typically had an audience with her new prime minister at Buckingham Palace.
She has been on a summer break at her Scottish home since July.
