
The Oxford University professor - who also wrote The Hobbit - was one of 50 intellectuals chosen by the British Government singled out to crack Nazi codes as it appeared increasingly likely Germany was preparing to declare war.
Tolkien was reknown as one of his generation's most respected linguists, and according to The Sun, was believed to have passed the training course "with flying colours".
He was offered the job at the famous Bletchley Park code-breaking centre, but for reasons unknown, turned it and its £50,000 ($94,000) salary down.
The staff at Bletchley Park would later achieve fame worldwide by cracking the Enigma code and saving Britain by helping its navy intercept and destroy Hitler's U-boats.
The Daily Telegraph reports Tolkien - a professor of Anglo Saxon at Oxford University from 1925 to 1945 - visited the base for three consecutive days in March 1939.
A record of his training carries the word ''keen'' beside his name, but he refused the job and went on to complete his Lord of the Rings trilogy.
"We simply don't know why he didn't join,” a spokesman for the UK Government's spy base said.
"Perhaps it was because we declared war on Germany and not Mordor."