The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has said.
All the Queen’s children are gathering at, or are travelling to, her Scottish estate near Aberdeen.
“Following further evaluation this morning, the Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision,” a statement said.
The Queen is “comfortable”, it added.
It is very unusual for Buckingham Palace to put out a statement like this – it is usually unwilling to provide a commentary on the 96-year-old monarch’s medical matters, which are seen as private.
Prince Charles has travelled to Balmoral with his wife, the Duchess of Cornwall.
The Duke of Cambridge, the Duke of York, and the Earl and Countess of Wessex arrived at Aberdeen airport just before 16:00 BST.
They are travelling to the Queen’s Scottish estate, about 40 miles (64km) west of Aberdeen.
The Princess Royal, Princess Anne, was already in Scotland undertaking engagements and is in Balmoral.
The Duchess of Cambridge has remained in Windsor with her children on their first full day at school.
The Duke and Duchess of Sussex, who were due to attend a charity event in London, are also travelling to Balmoral, said a spokeswoman.