The queen, Britain’s longest-serving sovereign and the world’s longest-serving monarch, has been suffering from what Buckingham Palace has called “episodic mobility problems” since late last year.
She has been forced to scale back her public engagements since then and on Wednesday canceled a planned virtual meeting with senior ministers after her doctors advised her to rest.
The day before, she had been photographed naming Liz Truss as the country’s new prime minister at her residence in Scotland, Balmoral Castle.
“The queen remains comfortable and at Balmoral,” the statement said.
A palace source said close relatives had been informed and played down speculation the monarch had suffered a fall.
Truss said the entire country would be deeply concerned by the news.
“My thoughts, and the thoughts of people across the UK, are with Her Majesty The Queen and her family at this time,” he said.
Lindsay Hoyle, Speaker of the House of Commons, interrupted an energy debate in Parliament to send her best wishes to the monarch.
With information from Reuters and AFP.