Chilling audio reveals Hitler's (pictured) unofficial tone for the first time after a conversation was secretly recorded of him speaking to Carl Mannerheim

A secret recording of Adolf Hitler talking privately with a military leader in 1942 uncovers the Nazi leader's normal speaking voice for the first time.

The chilling audio reveals Hitler's unofficial tone and is heard speaking privately with Carl Mannerheim without raising his voice.

The recording - made without the former First World War corporal knowing - shows his disillusionment when he finds out the Russian tanks production of 35,000 vehicles in the first year of the war.

And because Hitler never allowed recordings of him off-guard, it is the only known recording of Hitler not speaking in an official tone.

During the one-minute clip Hitler says: 'If somebody had told me a nation could start with 35,000 tanks, then I'd have said: "You are crazy!"'

He goes on to say 'right now' he has destroyed more than 34,000 tanks.

Finnish sound engineer Thor Damen, succeeded in recording the first eleven minutes of Hitler's and Mannerheim's private conversation.

Damen's original purpose was to record official birthday speeches and Mannerheim's responses but he decided to continue recording after the conversation switched from official to private.

It is believed the SS realised that Damen was recording the conversation, and they immediately demanded to have it stopped.

It was reported that the tape had been hidden away and was only made public in the 1950s.

And when the tape was revealed to the public, some believed it was a fake because Hitler's voice sounded too soft.

During the clip he speaks of his shock about a nation having 35,000 tanks.

He said: 'If you are one of my generals had stated that any nation has 35,000 tanks I'd have said: "You, my good sir, you see everything twice or ten times. You are crazy; you see ghosts."

'This I would have deemed possible. I told you earlier we found factories, one of them at Kramatorskaja, for example. Two years ago there were just a couple hundred [tanks].

'We didn't know anything. Today, there is a tank plant, where - during the first shift a little more than 30,000, and 'round the clock a little more than 60,000, workers would have labored - a single tank plant!'

The recording - made without the former First World War corporal knowing - shows his disillusionment when he finds out the Russian tanks production of 35,000 vehicles in the first year of the war. This file picture showsFile Hitler (left), listening as C.G.E Mannerheim (right)