Buckingham Palace has released a rare statement on the Queen's health as her family rush to be with her at Balmoral.
The Queen's close family including the Prince of Wales, his wife Camilla and Prince William have been informed, with the latest announcement escalating fears for the monarch's health.
Many of the top royals have arrived at Balmoral already, with some arriving by helicopter in the last hour including Princess Anne.
Prince Harry and wife Meghan are in the UK and are due to appear at the Well Child Awards this evening for sick children, but it is unknown if the Queen's health update will impact their plans.
The Royal spokesman 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."
Prime Minister Liz Truss said "the whole country will be deeply concerned by the news from Buckingham Palace this lunchtime".
She added "my thoughts - and the thoughts of people across our United Kingdom - are with Her Majesty The Queen and her family at this time".
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.
There was speculation on social media after politicians were seen passing notes in the House of Commons earlier today.
Speaker Lindsay Hoyle said to MPs: "We send our best wish to HM the Queen and that she and the Royal Family are in our thoughts and prayers at this moment."
The monarch, 96, has ongoing mobility issues and had been due to hold a Privy Council meeting virtually yesterday but it was postponed.
