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.”
The Queen’s close family have been informed, with the latest announcement escalating fears for the monarch’s health as family members cleared their diaries and headed to the Queen’s home in the Scottish Highlands.
The monarch has four children, eight grandchildren and 12 great-grandchildren – with all of the children thought to be on their way or already at Balmoral.
Prince Charles, the Prince of Wales, her eldest child and the heir to the throne is on his way to Balmoral along with the Duchess of Cornwall.
The Duchess of Cornwall had been scheduled to open a new visitor centre for the Great Tapestry of Scotland in Galashiels, in the Scottish Borders, on Friday.
Princess Anne, the Princess Royal, the Queen's second child and only daughter is also thought to be in Scotland.
Prince Andrew, the Duke of York, the Queen's third child who stepped down as a "working Royal" in 2019 after a controversial Newsnight interview is also on his way to Scotland.
Prince Edward, the Earl of Wessex and her youngest child is thought to be travelling to Her Majesty.
Prince William, the Duke of Cambridge, has also travelled to Balmoral today.
The Duchess of Cambridge has remained in Windsor as Prince George, Princess Charlotte and Prince Louis are on their first full day at their new school, Kensington Palace said.
The eldest son of Prince Charles and his first wife, Diana, Princess of Wales, is also thought to be on his way as is the Duke and Duchess of Sussex who will be travelling to Scotland according to a spokesperson for the couple.
The couple, who now live in the US, have been in the UK in recent days for various engagements.
Wellwishers have also gathered outside Balmoral
The monarch, 96, pulled out of a virtual Privy Council on Wednesday, a day after appointing Liz Truss as PM at Balmoral Castle.