The remarkable collection will feature as part of the Channel 5 documentary series Secrets of the Imperial War Museum, to be aired on Dec 17.

Alan Wakefield, the head of First World War at the museum, told The Telegraph: “We’ve got everything here – his letters, his course notebooks when he was training, photographs. We get offered lots of individual objects or collections. It’s impossible to take on everything. We’re trying to collect really exciting material like this collection.”

The donation was made by Dr Dean Clarke, a retired probation officer, who discovered the extraordinary mementoes when clearing out a house in Dore that had belonged to his step-father, Bernard, Malerbi’s son.

He said: “My stepfather died two years ago. I was left in lockdown sorting out his bungalow, getting rid of everything. That’s how I came across all this stuff. There was a huge number of suitcases in wardrobes and under beds with jaw-dropping letters.”

Malerbi wrote of close escapes and the dangers of venturing into No Man’s Land, the nightmarish wasteland between the enemies’ lines.

Mr Wakefield said: “He’s telling his parents how dangerous it is in the front line. He mentions decaying corpses in No Man’s Land in the trenches, the chance that you might be killed just walking down the trench by a stray artillery shell and the danger from snipers. He lays all that out. So I expect his parents were pretty worried.”

Malerbi, who died in the late 1970s and came from a comfortable Southampton family wrote: “We could smell the bodies in our trenches” and described the pain of losing friends.