The Queen is to rest for two more weeks on doctors' orders in the most significant absence of her long reign, and Buckingham Palace has confirmed she will miss an event linked to Remembrance Sunday.
The monarch, who has been having tests for an unknown illness, will not undertake any official visits in public and will be confined to desk-based duties only.
She is to miss the Festival of Remembrance on Saturday, November 13 but has the "firm intention" to be present to watch the annual wreath-laying on Remembrance Sunday.
A statement released by Buckingham Palace confirmed the Queen's doctors want her to rest for at least two more weeks.
The news will add to concerns about her health after she had to pull out of a trip to Northern Ireland and spent a night in hospital last Wednesday.
She later cancelled an appearance at the COP26 summit next week, where she had been determined to press world leaders to tackle the climate crisis.
While the Queen has been absent from public life due to ill-health before, when suffering heavy colds or recuperating from minor operations, the announcement she is going to take a further two weeks away from the public is unprecedented.
A spokesman said: "Following on from their recent advice that the Queen should rest for a few days, Her Majesty's doctors have advised that she should continue to rest for at least the next two weeks.
"The doctors have advised that Her Majesty can continue to undertake light, desk-based duties during this time, including some virtual audiences, but not undertake any official visits.
"Her Majesty regrets that this means she will be unable to attend the Festival of Remembrance on Saturday, Nov 13. However, it remains the Queen's firm intention to be present for the National Service of Remembrance on Remembrance Sunday, on Nov 14."
The Queen is continuing to undertake "light duties" at Windsor Castle, understood to mean telephone calls, virtual audiences and reading her red boxes. She was said to be in "good spirits" as she recorded her speech to the COP26 summit in Glasgow on Friday afternoon. It is due to be played to delegates on Monday.
A royal source said her diary for next week was light but she wanted to announce the need to take time off in good time to allow organisations hosting her to make plans. It is said to be a "sensible precaution" by doctors.
The cancellation of her attendance at the Festival of Remembrance is a major concession. The Queen lived through the war and makes the annual honouring of veterans a key part of her work.
The event is hosted by the Royal British Legion, and members of the British and Commonwealth Armed Forces past and present come together for musical performances and readings.
The Queen's presence would have been likely to require navigating stairs as well as being on show for the cameras for a prolonged period of time.
The more traditional Remembrance Sunday service could mean a more manageable appearance on a balcony to watch the wreath-laying, with simpler logistics to reduce the distance she had to walk or the length of time standing.
The Queen has recently been photographed using a walking stick for "comfort" and has been advised by doctors to give up drinking.
Nine days ago, it emerged that what was reported as tiredness requiring the cancellation of a flight to Northern Ireland was something more, requiring an overnight stay at the King Edward VII hospital in London for "preliminary tests".
A few days later, having taken the advice of specialist doctors, Buckingham Palace confirmed that Her Majesty would only be undertaking light duties inside Windsor Castle only.