The Queen’s immediate family members have been informed, with the latest announcement escalating fears for the monarch’s health.
The head of state pulled out of a virtual Privy Council on Wednesday after doctors ordered her to rest.
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.
Meghan Markle and Prince Harry are also on their way.
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's doctors are 'concerned' for her health PA
The Queen 'remains comfortable and at Balmoral' Wiki Commons
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."
The news was announced shortly after Ms Truss' energy price announcement in the House of Commons.
Speaker Sir Lindsay Hoyle interrupted the speech of SNP Westminster leader Ian Blackford during the debate to tell MPs: “I know I speak on behalf of the entire House when I say that we send our best wishes to Her Majesty the Queen and that she and the royal family are in our thoughts and prayers at this moment.”
He added: “If there is anything else, we will update the House accordingly.”
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”.
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."
The Royal Family’s website appeared to crash following the news of the Queen’s health concerns.
An error message appeared on screen reading: “Gateway time-out."
GB News' Nigel Farage commented: "Dreadful news from Buckingham Palace concerning the Queen’s health.
"The thoughts and prayers of the whole nation and huge numbers of people around the world are with her and her family at this time."