Queen Elizabeth II is “under medical supervision” at Balmoral, Buckingham Palace has said.
The decision was made by doctors who were “concerned” for the 96-year-old's health, following an assessment.
“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 spokesman for the palace said.
“The queen remains comfortable and at Balmoral.”
The announcement has set off a wave of concern across the UK, as the palace does not routinely issue updates on the sovereign's health and well-being.
The queen's daughter, Princess Anne, has already visited her mother while her three sons — Prince Charles, Prince Andrew and Prince Edward — rushed to be by her side on Thursday.
Prince Charles, heir to the throne, and his wife Camilla have travelled to Balmoral, their representatives at Clarence House said.
Prince Edward, his wife Sophie and Prince Andrew were also on their way.
Prince William is also en route to the Scottish castle, Kensington Palace confirmed. Palace officials added that his wife Kate had remained at their home in Windsor as their three children are on their first day at their new school. The palace earlier released photos of the couple arriving at the school with Prince George, Princess Charlotte and Prince Louis.
A flight containing members of the royal family was seen landing at Aberdeen International Airport on Thursday afternoon. Royal Air Force flight KRF23R took off from RAF Northolt in South Ruislip, west London, at 2.39pm, flight-tracking website Flightradar24 showed, and touched down in Scotland at 3.50pm.
Prince Harry and his wife Meghan, who stepped down from royal duties in 2020, are returning from their trip to Germany to be at his grandmother's side.
“The Duke and Duchess of Sussex will be travelling to Scotland,” a representative for the couple said. The couple had been due to attend the WellChild Awards ceremony in London on Thursday evening, but cancelled their scheduled appearances.
The Sussexes are on a whirlwind trip to Europe, having landed in the UK last week. They flew to Germany on Tuesday to help to launch the next year's Invictus Games. The couple returned to the UK in June for the queen's platinum jubilee and introduced their children Archie, 3, and 1-year-old Lilibet to their great-grandmother.
Robert Hardman, royal biographer and commentator, told BBC News: “The palace does not issue bulletins on the queen's health unless it is significant.”
The queen celebrated her platinum jubilee this year with a four-day national holiday that included concerts and church services. She is the only British monarch to reach the 70-year milestone.
She pulled out of some jubilee events, including a thanksgiving service at St Paul's Cathedral after experiencing “discomfort”, but she did make a starring appearance on the balcony at Buckingham Palace, flanked by members of her family, thrilling the tens of thousands of well-wishers gathered to mark the historic occasion.
The queen's most recent engagement was on Tuesday, when she welcomed Liz Truss to Balmoral, swearing her in as the new prime minister. She also received Ms Truss's predecessor, Boris Johnson, at the castle, where he officially offered his resignation.
Ms Truss on Thursday said the news would be a cause for concern across Britain.
“The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime,” she said.
“My thoughts — and the thoughts of people across our United Kingdom — are with Her Majesty the queen and her family at this time.”
Permanent secretaries from all government departments were due to attend an emergency meeting on Thursday afternoon to discuss a potential transition from one monarch to the next.
A sign reading “no guard changing ceremony today” was in position outside Buckingham Palace on Thursday morning but was later removed by officials.
In the last year, the queen has taken on fewer in-person events, relying on senior royals, including the Prince of Wales, to sit in for her. Prince Philip, her husband, died in April 2021.
The House of Commons speaker, Sir Lindsay Hoyle, said MPs across the political spectrum would “send our best wishes to Her Majesty the queen”. He added that “she and the royal family are in our thoughts and prayers at this moment”.
Labour leader Sir Keir Starmer said he was “deeply worried” by the news and hoped to see the queen make a full recovery.
“Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon,” the opposition leader tweeted.
The Archbishop of Canterbury also expressed his concern. “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.”
Former Labour prime minister Sir Tony Blair tweeted: “It is deeply concerning to hear today’s news from Buckingham Palace. My thoughts and prayers are with Her Majesty the queen and her family at this worrying time.”
The royal family's website appeared to crash following the news of the queen's health concerns. An error message appeared on screen reading: “Gateway time-out”.
