The monarch, 96, pulled out of a virtual Privy Council on Wednesday, a day after appointing Liz Truss as PM at her home in the Scottish Highlands.
A Palace spokesperson 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. The Queen remains comfortable and at Balmoral.’
Clarence House and Kensington Palace confirmed the two heirs to the throne were on their way to her bedside at the royal residence.
Harry and Meghan are also currently in the UK and were expected to attend the WellChild Awards in London this evening.
There has been no update from their office about how the development regarding Her Majesty’s health may affect their schedule.
The Queen was trending on Twitter following the news and the royal family’s website appeared to crash, with an error message on screen reading: ‘Gateway time-out.’
Prime Minister Liz Truss, who was briefed as she delivered a statement on the energy crisis in the Commons, said ‘the whole country will be deeply concerned by the news from Buckingham Palace this lunchtime’.
She added that ‘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’ by the news from the Palace and he is hoping for the Queen’s recovery.
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 Archbishop of Canterbury has said the ‘prayers of the nation’ are with the Queen.
‘My prayers, and the prayers of people across the (Church of England) and the nation, are with Her Majesty The Queen today,’ Justin Welby tweeted.
‘May God’s presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral.’
The nation’s longest-reigning monarch has ongoing mobility issues and looked bright but frail and used a walking stick during Tuesday’s historic audience with Ms Truss, which followed a visit from outgoing prime minister Boris Johnson as he tendered his resignation.
She has increasingly handed over duties to her heir, Prince Charles, and other members of the royal family in recent months as she has struggled to get around.