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The audience did not know who was about to appear on stage

Queen Camilla took to the stage at the Royal Festival Hall on Thursday evening in an unannounced appearance at the annual Letters Live event.

A hallmark of the format is that audiences are never told in advance who will appear or which letter they will read.

The Queen, who is patron of the National Literacy Trust, the charity the event supports, read a letter written in 1952 by the Queen Mother to the British poet Edith Sitwell.

The letter, dated September 15, 1952, was composed months after the death of King George VI and described how the grieving widow had found solace in an anthology of poetry that Ms Sitwell had sent her.

The choice of letter reflected the consoling and transformative power of literature, themes central to Queen Camilla's public work.

King George VI died in his sleep on February 6, 1952 following a prolonged illness, having reigned for 15 years.

His daughter Queen Elizabeth II, then 25, succeeded him immediately and was crowned the following year.

In the months that followed, Ms Sitwell sent the King's widow a collection of verse. The Queen Mother's reply, written that September, described taking the book to read beside a river on a day she felt overwhelmed by grief.

"It was a day when one felt engulfed by great black clouds of unhappiness & misery, and I found a sort of peace stealing round my heart as I read such lovely poems & heavenly words," the Queen Mother wrote to Ms Sitwell.

The Queen Mother's letter also quoted the poet George Herbert: "Who could have thought my shrivel'd heart, could have recovered greennesse? It was gone quite underground."

She reflected that sorrow felt "small and selfish" yet "bangs one about until one is senseless."

She thanked Ms Sitwell warmly for the gift, writing: "I can never thank you enough for giving me such a delicious book wherein I found so much beauty & hope quite suddenly one day by the river."

The letter praised the anthology as a remarkable gathering of rare and beautiful material, noting the effort that must have gone into assembling such variety.

The Queen Mother signed off by telling Ms Sitwell she was deeply moved by the gesture, adding: "I love being given books."

Letters Live was founded in London in 2013, drawing inspiration from Shaun Usher's bestselling Letters of Note series and Simon Garfield's To the Letter.

The event is produced by independent publisher Canongate in partnership with the film and television company SunnyMarch.

The series is approaching its 100th performance and has expanded internationally, raising funds for literacy charities in the UK and abroad.

Past performers have included Benedict Cumberbatch, Ian McKellen, Jude Law, Stephen Fry, Juliet Stevenson, Kylie Minogue and Sir Ben Kingsley, reading correspondence by figures ranging from David Bowie and Charlotte Brontë to Mohandas Gandhi and Che Guevara.