“Following further evaluation this morning, the Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision,” said Buckingham Palace in a statement.
“The Queen remains comfortable and at Balmoral.”
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.”
Labour leader Sir Keir Starmer said he is “deeply worried” after hearing the news from Buckingham Palace.
He tweeted: “Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
“My thoughts are with Her Majesty the Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery.”
The statement comes only 48 hours after the Queen met with Boris Johnson in Balmoral to accept his resignation as prime minister.
The monarch also met with Liz Truss on the same day and invited the new prime minister to form a new government.
It is understood Prince Charles and Camilla, Duchess of Cornwall are currently at the Queen’s side.
According to PA, the Princess Royal is at Balmoral, and the Duke of York and the Earl and Countess of Wessex are on their way to the Queen’s Scottish home, sources said.