Queen Elizabeth has been placed under medical supervision.
The 96-year-old monarch - who has been suffering with health issues and mobility difficulties in recent months - has left doctors "concerned" for her "health" but she is currently "comfortable" at her Balmoral estate in Scotland, where she has been on her annual summer break.
Buckingham Palace 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."
Her immediate family has been informed.
It is believed her grandson, Prince William, is travelling to Scotland at the time of writing, and her son Prince Charles and his wife Camilla, Duchess of Cornwall are already at Balmoral.
New Prime Minister Liz Truss said the "whole country" will be "deeply concerned" by the news.
She said in a statement: “The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime.
“My thoughts – and the thoughts of people across our United Kingdom – are with Her Majesty The Queen and her family at this time.”
During a debate on energy in the House of Commons, speaker Sir Lindsay Hoyle interrupted SNP's Westminster leader Ian Blackford to comment on the news.
He said: "I know I speak on behalf of the entire house when I say that we send our best wishes to Her Majesty the Queen, and that she and her family are in our thoughts and prayers at this moment.
"He says he is not going to say any more at this point."
The news comes after the queen postponed a meeting with the Privy Council via Zoom after being advised by doctors to rest.
A spokesperson said on Wednesday (07.09.22) evening: “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.”
During the planned Zoom proceedings, Mrs Truss would have taken her oath as First Lord of the Treasury and new cabinet ministers would have been sworn into their roles and made privy counsellors if not already appointed as one in past.
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