Buckingham Palace announced Thursday that Queen Elizabeth II is “under medical supervision” because “doctors are concerned” about her health.

“The Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision,” the palace said. “The Queen remains comfortable and at Balmoral.”

The 96-year-old monarch has spent the summer at the royal family’s estate in Aberdeenshire, Scotland, Balmoral Castle. On Thursday, a royal source told ABC News that immediate family members had been informed of the current situation.

According to their respective spokespersons, the queen’s oldest child, Charles, Prince of Wales, and his wife, Camilla, Duchess of Cornwall, as well as the queen’s daughter, Princess Anne, are already at Balmoral. Meanwhile, the queen’s other sons, Prince Andrew, Duke of York, and Prince Edward, Earl of Wessex, as well as Edward’s wife, Sophie, Countess of Wessex, are on their way to Balmoral, as are Charles’ sons, Prince William, Duke of Cambridge, Prince Harry, Duke of Sussex, and Harry’s wife, Meghan, Duchess of Sussex, according to their respective spokespersons.

According to an ABC News source, the queen canceled a meeting of her Privy Council on Wednesday and was told to rest. At the time, the palace would not comment on the queen’s health beyond saying that she was heeding doctors’ advice to rest.

Following the announcement from the palace on Thursday, British Prime Minister Liz Truss took to Twitter to express her “thoughts – and the thoughts of people across our United Kingdom – are with Her Majesty The Queen and her family at this time.”

“The news from Buckingham Palace this lunchtime will be deeply concerning to the entire country,” Truss tweeted.

Truss, who was recently chosen as the new leader of the ruling Conservative Party in the United Kingdom, met with the queen on Tuesday at Balmoral Castle, where she was formally asked to form a new government as prime minister following Boris Johnson’s official resignation. The ceremonial transfer of power was held at Balmoral for the first time in the queen’s 70-year reign, rather than at Buckingham Palace in London.

Over the last year, the queen has battled several health issues, including COVID-19 and an overnight hospitalization for what Buckingham Palace described as “preliminary investigations.”

Buckingham Palace changed the details of the queen’s job description in its annual report in July, according to the British newspaper The Telegraph, by swapping out the “Queen’s programme” of engagements for more general “visits in royal programmes.” Instead, members of the royal family, including Charles, the heir apparent to the British throne, will assist her with some of her responsibilities.

At the time, a palace source told ABC News that the changes were not “dramatic,” but rather a minor tweak. According to the Sovereign Grant report, the queen’s role still includes two key components: Head of State and Head of Nation, which means that as Head of State, she “must fulfill” specific duties.

Charles filled in for his mother in May when she was unable to attend the opening of Parliament due to mobility issues. It was the first time she hadn’t been present in six decades. Instead, her crown was placed alongside Charles’.