The Queen's family have been informed as doctors are concerned for Her Majesty's health.
The 96-year-old has been put under medical supervision after doctors voiced concern for the Queen.
Buckingham Palace issued a rare statement on the her health today to say that she remains comfortable and is staying at Balmoral.
Prince Charles and Camilla have travelled to Balmoral, Clarence House said.
Prince William has also raced to Scotland, Kensington Palace said.
Buckingham Palace issued a rare statement on the Queen's health today to say that she remains comfortable and is staying at Balmoral, and not in hospital.
The statement 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."
Moments before, MPs in the House of Commons appeared to look very concerned after the energy bill announcement.
There was speculation on social media after politicians were seen passing notes in the House of Commons earlier.
Prime Minister Liz Truss tweeted: "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."
Commons speaker Lindsay Hoyle said to 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."
The head of state pulled out of a virtual Privy Council on Wednesday after doctors ordered her to rest.
It comes just days after the Queen met with Liz Truss to officially appoint her as the new Prime Minister.
A Buckingham Palace spokesman said of the cancelling of the meeting: "After a full day yesterday, Her Majesty has this afternoon accepted doctors’ advice to rest.
"This means that the Privy Council meeting that had been due to take place this evening will be rearranged."
The nation’s longest-reigning monarch has ongoing mobility issues. She used a walking stick during Tuesday’s historic audience with Ms Truss, which followed a visit from former prime minister Boris Johnson.
