The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has 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," a statement said.
"The Queen remains comfortable and at Balmoral."
Prince Charles has travelled to Balmoral with the Duchess of Cornwall.
The Duke of Cambridge is on his way to the Scottish estate, about 40 miles (64km) west of Aberdeen.
The
announcement comes after the Queen, 96, pulled out of a meeting of her
Privy Council on Wednesday, with doctors advising her to rest.
There
are clearly pressing concerns for the Queen's health - much more
explicitly put than before and without any reference to this only being
about difficulties with mobility.
There
are also warnings against unfounded speculation, such as that she might
have had a fall. And on Tuesday she was on her feet and photographed
smiling as she appointed the new prime minister.
But
from the last-minute cancellation of what would only have been a
virtual meeting of the Privy Council - of senior ministers - there is no
mistaking the fragility of the Queen's health.
Prime Minister Liz Truss said the "whole country" would be "deeply concerned" by the news.
"My
thoughts - and the thoughts of people across our United Kingdom - are
with Her Majesty The Queen and her family at this time," she added.
The Queen appointed Ms Truss as prime minister at Balmoral, instead of travelling to London for the event.
During her 70-year reign the Queen has typically had an audience with her new prime minister at Buckingham Palace.
The
Archbishop of Canterbury, Justin Welby, said: "My prayers, and the
prayers of people across the [Church of England] and the nation, are
with Her Majesty The Queen today.
"May God's presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral."
Labour
leader Sir Keir Starmer said he was "deeply worried", while Scotland's
First Minister Nicola Sturgeon sent her thoughts and wishes to the
Queen.
Wales' First Minister Mark Drakeford sent his best wishes on behalf of the people of Wales.
The monarch has been on a summer break at her Scottish home since July.
