The announcement comes a day after the 96-year-old monarch cancelled a meeting of her Privy Council and was told to rest.
The palace says the queen is “comfortable" and remains at Balmoral Castle in Scotland, where she has spent the summer.
Doctors have recommended the 96-year-old remains under medical supervision. A palace source told Reuters that immediate members had been informed and played down speculation that the monarch had suffered a fall.
UK PM Truss says country 'deeply concerned'
UK Prime Minister Liz Truss tweeted that the whole country "will be deeply concerned" about the health of Queen Elizabeth II.
"The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime," Truss tweeted after the Palace issued a statement. "My thoughts -- and the thoughts of people across our United Kingdom -- are with Her Majesty The Queen and her family at this time," the UK PM added.
On Wednesday, the queen was forced to cancel a planned virtual meeting with senior ministers after being advised to rest by her doctors.
The previous day she had been pictured appointing Truss as the country's new prime minister at her Scottish home, Balmoral Castle.
