Buckingham Palace said the Queen was under medical supervision at Balmoral after doctors became concerned about her health.
All of the Queen’s children are either at, or moving to, her Scottish estate near Aberdeen.
“Following further assessment this morning, the Queen’s doctors are concerned for Her Majesty’s health and have recommended that she remain under medical supervision,” a statement said.
He added that the Queen is comfortable.
Prince Charles has traveled there with his wife, the Duchess of Cornwall.
The Duke of York and the Earl and Countess of Wessex are traveling to the Scottish estate, which is about 40 miles (64 km) west of Aberdeen.
The Princess Royal, Princess Anne, was already on a series of engagements in Scotland.
The Duke of Cambridge is also on his way.
The announcement comes after the 96-year-old Queen pulled out of a meeting of her Privy Council on Wednesday after doctors advised her to rest.
There are clearly pressing concerns about the Queen’s health – more clearly than ever and without reference to just mobility difficulties.
There are also warnings against unfounded speculation, such as its downfall. And on Tuesday she was on her feet and photographed smiling as the new prime minister was installed.
But the last-minute cancellation of what would have been only a virtual meeting of the Privy Council – of senior ministers – is no mistaking the fragility of the Queen’s health.
‘Deep Concern’
Prime Minister Liz Truss said “the whole country” would be “deeply concerned” by the news.
“My thoughts – and those of our people across the UK – are with Her Majesty the Queen and her family at this time,” he added.
The Queen appointed Ms Truss as Prime Minister at Balmoral rather than traveling to London for the event.
During her 70-year reign, the Queen has usually held an audience with her new prime minister at Buckingham Palace.
The Archbishop of Canterbury, Justin Welby, said: “My prayers, and those of the [Church of England] and people across the nation, are with the Queen today.
“May God’s presence strengthen and comfort His Highness, his family, and those who are caring for him at Balmoral”.
