The Queen is under “medical supervision” at Balmoral and doctors are concerned for her health, Buckingham Palace said.
The Palace issued a statement at 12.32pm on Thursday saying royal medics had concerns.
A 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 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.
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 nation’s longest-reigning monarch, 96, 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.
Labour leader Sir Keir Starmer said he was “deeply worried” by the news from Buckingham Palace and he is hoping for the Queen’s recovery.
Heir to the throne Charles was said to have been making regular morning visits to see his mother later in the summer as she continued to struggle with her mobility, with the unplanned visits considered highly unusual.
During her Platinum Jubilee celebrations, the Queen 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”.
Elizabeth II is the first British monarch in history to celebrate a Platinum Jubilee.
Her reign has stretched from the post-war years through a new millennium and into a radically altered 21st century.
In her twilight era, she has been setting her affairs in order, using her Jubilee message to endorse her daughter-in-law the Duchess of Cornwall, once a royal mistress, to be Queen and crowned at the Prince of Wales’s side when he one day becomes King.
She also faced a run of royal troubles in her Jubilee year.
She stripped her son the Duke of York of his military roles and he gave up his HRH style ahead of Andrew agreeing to pay millions to settle a civil sexual assault case.
More to follow.