The Queen is under medical supervision at Balmoral.
Prince Charles and Camilla, Duchess of Cornwall have travelled to Her Majesty's Highlands estate while Prince William is on his way.
Princess Anne is also there, while Prince Andrew, Prince Edward and Sophie, Countess of Wessex, are en route.
The news came shortly after Prime Minister Liz Truss' energy price announcement in the House of Commons.
A statement reads: “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.”
The Queen has had 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.
During her Platinum Jubilee celebrations, she only travelled to Buckingham Palace twice, first for her Trooping the Colour balcony appearance and then for a finale after the pageant.
She secretly spent a night in hospital in October undergoing tests and was then under doctors’ orders to rest for the next three months, missing the Remembrance Sunday Cenotaph service and Cop26 climate change talks.
The Queen caught Covid-19 in February, and suffered from mild cold-like symptoms but said the virus left her “very tired and exhausted”.
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's Sir Keir Starmer added: "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."
Prince Charles, Camilla and Prince William
PA
The Queen in the drawing room at Balmoral
PA