The British Prime Minister said the "whole country will be deeply concerned" by the news.
The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace said in a statement on Thursday.
It said in a
statement: "Following further evaluation this morning, The Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision. The Queen remains comfortable and at Balmoral."
Prime Minister Liz Truss tweeted soon after the news broke saying the "whole country will be deeply concerned".
https://twitter.com/trussliz/status/1567839967203794944
The announcement from the palace came after the Queen earlier postponed an online meeting of the Privy Council on Wednesday evening after being advised to rest by doctors.
The palace said that after "a full day" on Tuesday the monarch, 96, had accepted the medics' advice.
In a break in tradition she met outgoing Prime Minister Boris Johnson and his successor Liz Truss in Balmoral, in Aberdeenshire.
The Queen has suffered from mobility issues in recent times. The monarch remains at Balmoral and the latest advice does not involve a hospital stay.
She had been due to hold the Privy Council virtually on Wednesday evening, during which Truss would have taken her oath as First Lord of the Treasury and her new cabinet ministers would have been sworn into their roles.
In a statement a Buckingham Palace spokesman said: "After a full day yesterday, Her Majesty has this afternoon accepted doctors' advice to rest.
"This means that the Privy Council meeting that had been due to take place this evening will be rearranged."
