Queen Elizabeth II waits in the Drawing Room before receiving Liz Truss for an audience at Balmoral, Scotland, Britain 06 September 2022. Truss was in Balmoral for an audience with Queen Elizabeth II where she was invited to become Prime Minister and form a new government. EPA-EFE/Jane Barlow / POOL 
The queen, Britain's longest reigning sovereign and the world's oldest monarch, has been suffering from what Buckingham Palace has called "episodic mobility problems" since the end of last year.
She has been forced to cut back on her public engagements since then and on Wednesday cancelled a planned virtual meeting with senior ministers after being advised to rest by her doctors.
The previous day she had been pictured appointing Liz Truss as the country's new prime minister at her Scottish home, Balmoral Castle.
"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 palace said in a statement.
"The queen remains comfortable and at Balmoral."
A palace source said immediate members had been informed and played down speculation that the monarch had suffered a fall.
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 said.
Lindsay Hoyle, the speaker of the House of Commons, interrupted an energy debate in parliament to say he sent his best wishes to the monarch.
(Reporting by Michael Holden; Editing by Kate Holton)