Doctors are "concerned" for the health of Queen Elizabeth II and have placed her under medical supervision, Buckingham Palace announced Thursday.
"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 Palace said in a statement. "The Queen remains comfortable and at Balmoral."
The Queen's eldest son and heir Prince Charles is with her, while other family including Princes William and Harry have also journeyed to Scotland.
Prime Minister Liz Truss said in a statement, "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," Truss said.
The Queen was last seen publicly on Tuesday at her Scotland residence, where she formally invited Truss to become prime minister. In photos of the meeting, the 96-year-old monarch used her cane and smiled at cameras.
Such meetings usually take place at Buckingham Palace in London, but Truss and outgoing prime minister Boris Johnson took the unusual step of traveling to Scotland because of the monarch's mobility issues.
On Wednesday, the Queen was advised by doctors to rest and to delay a meeting of the privy council.
UK Opposition Leader Keir Starmer said Thursday he was "deeply worried" by the news from the royal family. "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," Starmer said.
In October last year, the Queen canceled a trip to Northern Ireland on medical advice. She also spent one night in a London hospital.
Since then, her public events have been extremely limited.
In June, the Queen marked her Platinum Jubilee, with celebrations taking place across the UK in recognition of her 70 years on the throne.
Ellie Hall contributed reporting.