The Queen is under medical supervision at Balmoral after doctors became concerned for her health, Buckingham Palace has said.
Prince Charles and Camilla are both at the Scottish estate, Clarence House confirmed. The Princess Royal is also at Balmoral.
The Duke of Sussex is travelling to Scotland, a spokesperson for the couple said.
Her Majesty, 96, postponed her audience with the Privy Council on Wednesday on doctors orders.
The day before the monarch swore in new Prime Minister Liz Truss in Scotland, in a break from tradition, amid ongoing issues with her mobility in her advanced age.
The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime.
— Liz Truss (@trussliz) September 8, 2022
My thoughts - and the thoughts of people across our United Kingdom - are with Her Majesty The Queen and her family at this time.
Speaker Lindsay Hoyle interrupted a House of Commons debate on the energy crisis on Thursday to inform MPs about the monarch’s health.
He said: “I know I speak on behalf of the entire House when I say that we send our best wishes to Her Majesty the Queen and that she and the royal family are in our thoughts and prayers at this moment.”
Just before 4pm Prince William, Prince Andrew, Prince Edward and Sophie Wessex landed on an RAF plane at Aberdeen airport.
Barriers are being put up around the Balmoral estate as wellwishers begin to gather at the gates.
BBC One has suspended its regular programming schedule until at least 6pm.
A crowd gathered outside Buckingham Palace beneath dark clouds as news spread that the Queen is under medical supervision at Balmoral.
More than 100 umbrella-clad observers congregated on the stone steps up to The Queen Victoria Memorial, opposite the royal residence, and dozens more stood beside the gates.
Despite the size of the crowd, the atmosphere was relatively quiet and many peered through the bars of the main gates.
Liz Truss said: “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.”
Labour leader Keir Starmer also said he was “deeply worried” about the news.
Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
— Keir Starmer (@Keir_Starmer) September 8, 2022
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.
A Buckingham Palace spokesman said: “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 Queen remains comfortable and at Balmoral.”
The Archbishop of Canterbury has said the “prayers of the nation” are with the Queen.
“My prayers, and the prayers of people across the (Church of England) and the nation, are with Her Majesty The Queen today,” Justin Welby tweeted.
“May God’s presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral.”
The Duchess of Cornwall had been scheduled to open a new visitor centre for the Great Tapestry of Scotland in Galashiels, in the Scottish Borders, on Friday. That is now likely to be cancelled.
The Duke and Duchess of Sussex were due to attend the WellChild Awards ceremony in London on Thursday evening, but have changed their plans to travel to see the Queen.
