Prince Charles and William are travelling to Balmoral to visit the Queen who is under medical supervision.
In a statement, the Palace said the Queen was 'comfortable'.
It read: "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."
The Queen’s close family including the Prince of Wales have been informed, with the latest announcement escalating fears for the monarch’s health.
Prime minister Liz Truss said: "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."
Commons Speaker Sir Lindsay Hoyle told MPs: "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 the royal family are in our thoughts and prayers at this moment."
Labour leader Sir Keir Starmer said he was 'deeply worried' by the news from Buckingham Palace and that he was hoping for the Queen’s recovery.
He said in a tweet: "Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
"My thoughts are with Her Majesty the Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery."
Following today's announcement, the royal family’s website appeared to crash, with an error message appeared on screen reading, 'Gateway time-out'.
There have been concerns over the Queen's health for some time, with her drawing back from duties over recent months.
She appeared with a walking stick when she greeted Boris Johnson and Liz Truss on Tuesday (6 September), to formally announce the change of leadership.
Following the meeting, it was revealed that the Queen also postponed a virtual meeting of the Privy Council.
A Buckingham Palace spokesperson said: “This means that the Privy Council meeting that had been due to take place this evening will be rearranged.”
"After a full day yesterday, Her Majesty has this afternoon accepted doctors’ advice to rest."
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