Queen Camilla is missing the Royal Variety Performance on Friday, following "lingering post-viral" symptoms from a recent chest infection.
Buckingham Palace said doctors have advised the Queen - who is 77 - to prioritise rest as she continues to recover from her infection that resulted in her pulling out from many scheduled events including Remembrance Sunday.
She returned to public duties last week, but is understood to still be under the weather after a busy week of engagements.
On Wednesday the Queen appeared at an event at the University of London, where she was presented with an honorary doctorate of literature by Princess Anne.
A Palace source said the Queen is disappointed to be missing the annual event at the Royal Albert Hall in London, but that she "is a great believer that 'the show must go on'".
"She hopes to be back to full strength and regular public duties very soon," the source added.
King Charles III attended the show as planned.
Friday's performance is being presented by Amanda Holden and Alan Carr, and will see Sir Elton John and his husband David Furnish present a musical number from their new show The Devil Wears Prada.
The Queen's return to public duties had been adjusted to avoid any setback to her recovery.
Other events she attended this past week include a reception she hosted at Clarence House for authors shortlisted for this year’s Booker Prize, and a Buckingham Palace reception to celebrate the TV and film industry.
Both engagements were shorter than originally planned.
The Queen's chest infection came just under a week after returning from a trip to Australia and Samoa with the King. The trip included a stopover in India on the way back.
Announcing the infection earlier this month, Buckingham Palace said: "Her Majesty The Queen is currently unwell with a chest infection, for which her doctors have advised a short period of rest."
It said she had cancelled her engagements "with great regret".
The Queen previously missed a week of engagements with ill health in February 2023, when she tested positive for Covid. It was the second time she contracted the virus, having previously had it the year before.
