The Queen is unwell with a chest infection and has withdrawn from her engagements this week, Buckingham Palace has announced.
Camilla, 77, who has not long returned from a busy official overseas tour to Australia and Samoa, is understood to be resting at home.
The update comes after a year of difficult health troubles for the royal family, with both the King and the Princess of Wales being diagnosed with cancer.
Prime Minister Sir Keir Starmer wished Camilla a “speedy recovery” on behalf of the whole country, in a get well message on social media.
A Palace spokesman said: “Her Majesty The Queen is currently unwell with a chest infection, for which her doctors have advised a short period of rest.
“With great regret, Her Majesty has therefore had to withdraw from her engagements for this week but she very much hopes to be recovered in time to attend this weekend’s Remembrance events as normal.
“She apologises to all those who may be inconvenienced or disappointed as a result.”
The Duchess of Gloucester has stepped in to attend the annual opening of the Field of Remembrance at Westminster Abbey on Thursday on the Queen’s behalf.
Camilla has also pulled out of attending the Buckingham Palace reception for Olympic and Paralympic athletes hosted by the King the same evening.
No further details regarding her illness or treatment have been released, but she is understood to be being cared for under doctors’ supervision.
Meanwhile, the King was carrying on with his duties at Buckingham Palace on Tuesday, holding audiences with two new ambassadors and a high commissioner.
The 75-year-old monarch welcomed Qatari ambassador Sheikh Abdulla bin Mohammed bin Saud Al-Thani; Rajendre Khargi, who is the ambassador from the Republic of Suriname; and the High Commissioner for the Republic of Vanuatu, Georges Maniuri, who presented their credentials in the 1844 room.
Charles and Camilla spent around two weeks out of the UK for their nine-day official tour.
They stopped off in India for a luxury spa break on their return flight from Samoa, with the King following advice to take periods of rest to prioritise his cancer recovery.
The royal couple were reported to have spent three days at the £3,000-a-week holistic Soukya resort in Bangalore, known for its yoga programmes and Ayurvedic treatments.
The Queen has been focused on her husband’s health in recent months, telling author Lee Child in June that he was “doing fine” but “won’t slow down and won’t do what he’s told”.
Charles, who was diagnosed with an undisclosed form of cancer in February, paused his treatment to undertake his historic first official trip to Australia as the country’s monarch in October.
The visit hit the headlines when Australian senator Lidia Thorpe berated the King and accused him of “genocide” against the First Nations, after his speech during the monarch and Camilla’s official welcome to Canberra.
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