The Queen has been under medical supervision from doctors following a Palace announcement revealing concerns for her health earlier today.
A Buckingham Palace spokesperson 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.
“The Queen remains comfortable and at Balmoral.”
Other senior royals have since gone to Balmoral, with Princes William, Andrew and Edward arriving at the Scotland estate later in the afternoon.
Catherine, Duchess of Cambridge, has reportedly remained in Windsor. Their three children have just started at a new school.
Crowds have gathered outside Buckingham Palace.
Prime Minister Liz Truss tweeted: “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.”
The Queen formally appointed Truss as prime minister at Balmoral on Tuesday instead of Buckingham Palace – where prime ministers are usually instated.
Labour leader Keir Starmer tweeted: “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.”
Speaker of the House of Commons Lindsay Hoyle 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 the Royal Family are in our thoughts and prayers at this moment.”
The Queen celebrated her Platinum Jubilee this summer, marking 70 years on the throne. She is 96 years old.