The files had survived a post-war order to burn all evidence of MI7b because a colleague of Milne, Captain James Lloyd, took them home, the Daily Express reported.
They were forgotten about until they were unearthed by the captain’s great-nephew, Jeremy Arter.
He realised their significance while clearing the house in Brecon, Mid-Wales.
“Much of the household belongings were to go in a skip. I was about to throw everything away but, leafing through, I saw a book with MI7b written on it and decided to take a closer look. When I turned the cover and saw the name AA Milne, I knew it was a historic document,” Arter said.