Queen Elizabeth II's doctors are "concerned" for her health, Buckingham Palace have announced.
The 96-year-old British monarch has been placed under medical supervision at Balmoral Castle in Scotland.
The palace said she "remains comfortable".
Britain's Prime Minister Liz Truss said the "whole country will be deeply concerned by the news from Buckingham Palace."
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."
Prince Charles and Prince William are travelling to Balmoral to see her - as is Prince Harry.
She had been due to hold a meeting of senior government advisers virtually on Wednesday evening, but it was decided it would be rearranged.
Ms Truss currently has no plans to travel to Scotland today or tomorrow, Downing Street said.
Other key figures including former British prime ministers Tony Blair and David Cameron, Canadian premier Justin Trudeau and Scottish First Minister Nicola Sturgeon have all expressed their concerns for her.
Queen Elizabeth has faced ongoing "episodic mobility problems" since last autumn, and has been relying on a walking stick.
She also missed the state opening of the British parliament for the first time in 59 years in May - with Charles reading her speech instead.
And she was forced to miss some celebrations for her Platinum Jubilee back in June.
Her role was formally re-written by Buckingham Palace to scale back her "official duties" in July, for the first time in more than a decade.
She arrived at Balmoral for her summer break on July 21st.
Additional reporting: Jack Quann/IRN
Main image: Britain’s Queen Elizabeth after attending the opening ceremony of the sixth session of the Senedd in Cardiff, Wales, 14-10-2021. Image: