The letters - about 1.000 have survived - were sent to London from just after the French surrender to Germany in June 1940, through to the end of 1943.

They were addressed to the French service of the BBC, otherwise known as Radio Londres, which during the German occupation was a vital source of information and comfort for millions of French men and women.

Extracts from the letters were read out on Friday evenings on a programme called The French Speak to the French, whose aim was to build morale and stiffen civilian resistance to the Germans and Vichy.

After the war, the letters were put in storage and forgotten. That was until historian Aurelie Luneau stumbled upon them while researching her thesis on Radio Londres.

"I was in this tiny room in the BBC archives in Reading, and they brought me a box marked 'Letters from France'," she says. "One look inside and I knew it was one of those finds that historians normally only dream about."