Queen Elizabeth II left a sealed letter for her son and heir in her final red box, according to a new biography.

The late Queen included a private letter for the new King in her final set of signed papers, brought down by staff from her deathbed at Balmoral.

A new biography of King Charles III, giving detail of Elizabeth II’s final hours for the first time, describes the box as the “last completed homework of the longest reign in history”.

The red boxes are one of the monarch’s most famous duties, completed on every day except Christmas Day and Easter Sunday.

The late Queen can now be revealed as working her way through them diligently even as she was confined to her bedroom.

Writing of the aftermath of the Queen’s death, on September 8, 2022, biographer Robert Hardman describes how, as private secretaries Sir Edward Young and Sir Clive Alderton settled down to work their way through official business, a “footman appeared with a red box”.

“It was the last one that had gone up to the Queen before her death.”

Saying Sir Edward was “not sure what to expect as he turned the lock”, he writes: “Inside, he found that Elizabeth II had left a sealed letter to the Prince of Wales and a private letter to himself.

“Were they final instructions or final farewells? Or both?

“We will probably never know what they said. However, it is clear enough that the Queen had known that the end was imminent and had planned accordingly.”

The last document ever handled by the late Queen
The box also included the late Queen’s approved shortlist of candidates to receive the Order of Merit, intended to honour distinguished service in public life and in the gift of the monarch.

“It was the last document ever handled by Queen Elizabeth II,” Hardman writes.

“Even on her deathbed, there had been work to do. And she had done it.”

The biography also describes how the late Queen had seemed “energised” by a win for her horse Love Affairs at Goodwood on September 6, appearing “buzzy” over pre-dinner drinks and recalling the Prime Ministers she had known.

She decided to have dinner alone upstairs, feeling tired after holding audiences with the outgoing and incoming Prime Ministers and posing for a final photograph.

The next day, aides made arrangements for a Privy Council meeting to be heard virtually with an audio-only connection to London where politicians were assembling in person.

It was cancelled on medical advice only at the point the meeting was meant to go ahead, it is reported.

The book, Charles III by Robert Hardman, is out in hardback on January 18, published by Macmillan. It is subtitled: “New King. New Court. The Inside Story.”