London: It was one of the most notorious stories that Germany's Nazi dictator Adolf Hitler used to say -- his First World War experience ended when he was blinded by a British mustard gas attack.

Now, a new research has exposed Hitler's account of his own gallantry as a sham, and revealed that his temporary loss of sight was caused by a mental disorder known as "hysterical blindness" and not the mustard gas attack he alleged.

Hitler had claimed numerous times during his lifetime, including in his autobiography 'Mein Kampf’ that the British attacked in October 1918 south of Ypres using a "yellow gas unknown to us". And, by morning, his eyes "were like glowing coals, and all was darkness around me", he wrote in his book.

But, historian Dr Thomas Weber of University of Aberdeen,says he has uncovered a series of unpublished letters between two American neurologists from 1943, which debunk Hitler's claim, the 'Daily Mail' reported.

The correspondence showed that Otfried Foerster, a renowned German neurosurgeon, had inspected Hitler's medical file, and found that he was treated for hysterical amblyopia, a psychiatric disorder that can make sufferers lose sight.

Dr Weber said: "There were rumours suggesting that his war blindness may have been psychosomatic, but this is the first time we have had any firm evidence."

He said discovering the letters was "crucial" because Hitler's medical file, at the Pasewalk military hospital in Germany, was destroyed. "Hitler went to extreme lengths to cover up his First World War medical history," Dr Weber said.

Dr Weber said the letters could help to explain Hitler's radical personality change after the war. "Hitler left First World War an awkward loner who had never commanded a single other soldier, but very quickly became a charismatic leader who took over his country," he said.

He added: "His mental state could explain this dramatic change and his obsessive and extreme behaviour. The evidence also gives a crucial insight into Hitler's mental state during his leadership."