Audience members at a packed central London show were greeted with an unexpected appearance when the Queen took to the stage to deliver a reading.
Camilla led the line-up for Letters Live at the Royal Festival Hall, an event famed for unannounced celebrity appearances reciting monologues from historical correspondence.
Her arrival prompted gasps from the crowd before a deafening applause broke out across the venue.
While Thursday night’s star-studded show included readings from actors Jessie Buckley, Olivia Colman and Benedict Cumberbatch, attendees appeared notably excited by the royal contributor.
Camilla read a letter from the late Queen Mother to poet Edith Sitwell, in which she thanked Ms Sitwell for an anthology of poems sent to comfort her following the death of her husband, King George VI.
Written on 15 September 1952, the note from Charles’s maternal grandmother detailed how she found solace in the collection as she mourned the late King, who died that February.
Camilla, an avid reader who champions the importance of literacy in much of her work, read to the captivated audience at the Royal Festival Hall, saying: “My dear Miss Sitwell, it was so very kind of you to send me a copy of your lovely book.
“It is giving me the greatest pleasure, and I took it out with me and I started to read it, sitting by the river and it was a day when one felt engulfed by great black clouds of unhappiness and misery.
“I found a sort of peace stealing round my heart as I read such lovely poems and heavenly words.”
The letter Camilla read concluded: “I am deeply touched by your thought of me, I love being given books, and I send you my warmest thanks.
“I am, yours very sincerely, Elizabeth R.”
Olivier-award winner Simon Callow, Law & Order: UK star Dame Harriet Walter and TV barrister Rob Rinder were also on the surprise line-up.
On Wednesday, the Queen visited the Bayeux Tapestry alongside the King, the prime minister and the French president to mark the historic return of the centuries-old artwork to the UK.
In a public display of warm Anglo-French relations, Charles and Camilla joined Andy Burnham, Emmanuel Macron and their wives as they walked the length of the 70-metre masterpiece at the British Museum in London.
