Queen Elizabeth's health is causing "concern" for her doctors, Buckingham Palace said in a statement 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 read. "The Queen remains comfortable and at Balmoral."
The 96-year-old monarch's immediate family has been informed, a royal source says.
"Their Royal Highnesses The Prince of Wales and the Duchess of Cornwall have traveled to Balmoral," Buckingham Palace said, adding that Prince William is also making his way to Balmoral from his home in Windsor.
Prince Harry and Meghan Markle, who are currently in the U.K. for a series of charity events, are traveling to Scotland, according to a representative for the couple.
Princess Anne, Prince Albert, Prince Edward and wife Sophie, Countess of Wessex will also be by their mother's side in Balmoral.
The news follows an announcement Wednesday that the Queen would not preside over a scheduled Privy Council meeting so she could rest.
Jane Barlow - WPA Pool/Getty Queen Elizabeth on Sept. 6, 2022
"After a full day yesterday, Her Majesty has this afternoon accepted doctors' advice to rest," the palace said in Wednesday's statement. "This means that the Privy Council meeting that had been due to take place this evening will be rearranged."
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On Tuesday, the Queen welcomed Conservative Party leader Liz Truss to her retreat in the Scottish Highlands and bid outgoing Prime Minister Boris Johnson a formal farewell.
Andrew Milligan/Pool/Getty Queen Elizabeth
In a statement on Thursday, 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."
Queen Elizabeth has been using a cane regularly since last October and has been experiencing ongoing mobility issues. The same month, she canceled a scheduled trip to Northern Ireland under medical advice from her doctors and spent a night in the hospital.
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Jane Barlow/AP/Shutterstock Queen Elizabeth and Liz Truss
The Queen also decided not to appear at the Remembrance Day ceremony in November due to a sprained back and opted not to celebrate a traditional Christmas with the royal family at Sandringham, partially due to the uptick in COVID-19 cases around the holidays.
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Hannah McKay/getty Prince Charles and Queen Elizabeth in June
In February, the palace announced that Queen Elizabeth had tested positive for COVID-19. "Her Majesty is experiencing mild cold-like symptoms but expects to continue light duties at Windsor over the coming week," the palace said.
The monarch made three appearances for her Platinum Jubilee weekend in June but did skip a few events.
Her last public outing was in July, when she stepped out with daughter Princess Anne to open the new 28-bed Thames Hospice building. There, the Queen animatedly chatted with patient Pat White, also speaking with staff and volunteers during a tour.