Harry is heading to Scotland on Thursday, where the monarch is under medical supervision at Balmoral Castle, a spokesperson for the Duke of Sussex tells PEOPLE.
Harry, 37, and Meghan, 41, were scheduled to attend the WellChild Awards in London on Thursday, but their appearance has now been canceled. They traveled from their California home to Europe this week for a series of visits with their longstanding charitable organizations.
Buckingham Palace said in a statement Thursday morning, "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."
Samir Hussein/WireImage Meghan Markle and Prince Harry
Prince Harry joins family members in rushing to Queen Elizabeth's side. The Queen's son and heir, Prince Charles, and his wife, Camilla, Duchess of Cornwall, have traveled to Balmoral. Prince Charles was in Scotland on Wednesday carrying out engagements.
Prince William, Harry's brother who is second in line to the throne, also headed to Scotland on a plane with Queen Elizabeth's other two sons, Prince Andrew and Prince Edward. Edward's wife, Sophie, Countess of Wessex, was also onboard.
Princess Anne, the Queen's only daughter, is already in Scotland having undertaken royal duties in the country this week.
Jane Barlow - WPA Pool/Getty Queen Elizabeth on Sept. 6, 2022
Truss tweeted in reaction to the palace's statement on Thursday, "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."