The Queen is under medical supervision at Balmoral after doctors became concerned for her health.
A spokesperson at Buckingham Palace announced the news on Thursday afternoon. They added she 'remains comfortable' at this time.
The Prince of Wales and Duchess of Cornwall has travelled to Balmoral, Clarence House has said - as will the Duke of Cambridge, according to Kensington Palace.
A 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."
Following the news, 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 said the Queen was 'in the thoughts and prayers' of the entire house as he sent their best wishes.
He 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.”
Scotland's First Minister Nicola Sturgeon said: "All of us are feeling profoundly concerned at reports of Her Majesty's health. "My thoughts and wishes are with the Queen and all of the royal family at this time."
Holyrood's Presiding Officer has said the thoughts of all in the Scottish Parliament are with the Queen after Buckingham Palace said she was under medical supervision at Balmoral.
Speaking after First Minister's Questions, Holyrood Presiding Officer Alison Johnstone made a brief statement to MSPs.
"I am aware that a statement has been made at Westminster on the health of Her Majesty the Queen," she said.
"I will, of course, monitor developments and keep members updated over the course of the day.
"But my, I'm sure the thoughts of all in Parliament are with Her Majesty at this time."
The Queen's immediate family members have been informed, with the latest announcement escalating fears for the monarch's health. The head of state pulled out of a virtual Privy Council on Wednesday after doctors ordered her to rest.
It comes just two days after the Queen was pictured in a historic audience with Liz Truss, as she appointed her as the new Prime Minister.
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