Queen Elizabeth is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has said, BBC reported.
All the Queen's children are either 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, BBC reported.
Prince Charles has travelled to Balmoral with his wife, the Duchess of Cornwall.
The Duke of York and the Count and Countess of Wessex are travelling to the Scottish estate, about 40 miles (64km) west of Aberdeen.
The Princess Royal, Princess Anne, was already in Scotland undertaking engagements.
The Duke of Cambridge is also on his way, with the Duchess of Cambridge remaining in Windsor with their children on their first full-day at school.
Barriers are being placed outside one of the entrances to the estate.
There are clearly pressing concerns for the Queen's health - much more explicitly put than before and without any reference to this only being about difficulties with mobility.
There are also warnings against unfounded speculation, such as that she might have had a fall. And on Tuesday she was on her feet and photographed smiling as she appointed new Prime Minister Liz Truss, BBC reported.
But from the last-minute cancellation of what would only have been a virtual meeting of the Privy Council - of senior ministers - there is no mistaking the fragility of the Queen's health.