Queen Elizabeth II is under medical supervision as doctors are '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 the Privy Council on Wednesday, with doctors advising her to rest.
There are clearly pressing concerns for the Queen's health - much more explicitly put than before and without any reference to this only being about difficulties with mobility.
There are also warnings against unfounded speculation, such as that she might have had a fall. And on Tuesday she was on her feet and photographed smiling as she appointed the new prime minister.
But from the last-minute cancellation of what would only have been a virtual meeting of the Privy Council - of senior ministers - there is no mistaking the fragility of the Queen's health.
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.
The Queen appointed Ms Truss as prime minister at Balmoral, 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.
Labour leader Sir Keir Starmer said he was "deeply worried" by the news, while Scotland's First Minister Nicola Sturgeon said she was "profoundly concerned" and sending her thoughts and wishes to the Queen.
Wales' First Minister Mark Drakeford sent his best wishes on behalf of the people of Wales.