Queen Elizabeth II is under medical supervision after doctors for the 96-year-old monarch became "concerned for her health," Buckingham Palace said in a statement on Thursday.
“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 statement said.
The statement added that the queen "remains comfortable" at Balmoral, the royal castle in Scotland that serves as her summer retreat.
The queen's four children as well as grandchildren Prince William and Prince Harry are traveling to Balmoral to be with her, according to multiple spokespeople for them.
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Her son, Prince Charles, and his wife, Camilla, Duchess of Cornwall, have travelled to Balmoral along with Prince William, according to spokespeople for Clarence House and Kensington Palace.
William's wife, Catherine, Duchess of Cambridge, has remained in Windsor because Thursday is the first full day of school for their children, Prince George, Princess Charlotte and Prince Louis, a Kensington Palace spokesperson told the UK Press Association.
William's brother, Prince Harry, and Meghan, Duchess of Sussex, are also traveling to Balmoral, according to a spokesperson for the royal couple.
The queen's three other children, Anne, Princess Royal, Prince Edward and Prince Andrew, are either traveling to Balmoral or already there with the queen.
British Prime Minister Liz Truss tweeted her well wishes for the queen on Thursday, saying the whole country "will be deeply concerned" about the news of her health.
"The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime," she wrote. "My thoughts — and the thoughts of people across our United Kingdom — are with Her Majesty The Queen and her family at this time."
The queen met with Truss at Balmoral on Tuesday to formally appoint her as the country's new prime minister in the ceremonial transfer of power. This was the first time in the queen’s 70-year reign that the handover of power took place someplace other than Buckingham Palace in London.
The queen was not scheduled to make any public appearances on Thursday but did cancel a virtual meeting on Wednesday night with the Privy Council, which is her formal body of advisers.
Thursday's announcement also comes less than a week after she canceled her annual appearance at a sporting event in Scotland due to mobility issues.
This is a developing story, check back for updates.