The contents of the file, which has potentially not seen the light of day for at least 30 years, date from early 1949 until Turing's death in June 1954, said researchers at the University of Manchester, where the letters were found.
The letters provide a unique glimpse into Turing's every day working life and some of his more forthright personal opinions such as his dislike for America.
"I would not like the journey, and I detest America," Turing wrote in one of the letters.
The 148 documents include a handwritten draft for a BBC radio programme about Artificial Intelligence (AI) and offers to lecture from some of America's most famous universities, such as Massachusetts Institute of Technology (MIT).
A lot of the letters focus on Turing's research and his forward, ground-breaking thinking in areas such as AI, computing and mathematics.
"When I first found it I initially thought, 'that can't be what I think it is', but a quick inspection showed it was, a file of old letters and correspondence, by Alan Turing," said Jim Miles, professor at University of Manchester, who found the letters.
"I was astonished such a thing had remained hidden out of sight for so long. No one who now works at the university knew they even existed. It really was an exciting find and it is mystery as to why they had been filed away," Miles added.
The collection was initially found in May this year, but has now been sorted, catalogued and stored at the university's library.
"This is a truly unique find. Archive material relating to Turing is extremely scarce, so having some of his academic correspondence is a welcome and important addition to our collection," said James Peters, archivist at University of Manchester.
"There is very little in the way of personal correspondence, and no letters from Turing family members. But this still gives us an extremely interesting account and insight into his working practises and academic life whilst he was at the University of Manchester," he added.
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