THE story of how a maverick aristocratic Scot founded one of the world’s most famous military units has been clouded in secrecy for more than 70 years.

Now the story of how the SAS came into being in the north African desert in 1941 is being revealed after the special forces unit agreed to open up its archives for the first time.

The makers of a new BBC Two documentary series have been given access to unseen material – such as a scrapbook of combat reports detailing every mission – and interviews with its founding members which have been hidden away for decades.

The man who came up with idea of forming a small specialised unit which could get behind enemy lines to attack was Sir David Stirling, who was born in Perthshire. At the time, he was an ordinary lieutenant described as so tall and lazy his colleagues had nicknamed him the “Giant Sloth”.

But with Britain losing the war, Stirling came up with a plan for it to form a paratrooper regiment – which the Germans already had. However his initial attempts to test out the idea ended in disaster, after he acquired a parachute and jumped out of a plane with no training whatsoever.

The programme shows excerpts from an interview recorded with Stirling in 1987, just three years before his death at the age of 74, in which he described the accident which followed.

He said: “I was a bit unlucky because my parachute, when it opened, was attached to the tailplane and before it broke loose it took off a panel or two off the parachute. Of course I descended a good deal faster than my companions.

“I couldn’t move either of my two legs and went to Alexandria hospital [in Egypt] and of course it gave me a marvellous opportunity to do some work on the project.”

Undaunted, he persuaded senior officers – after breaking into the British HQ in Cairo on crutches to present his plans – to allow him to form a band of six officers and 60 men who could launch surprise attacks by parachuting in behind enemy lines.

Stirling described the first men who joined up as “band of vagabonds”.

He said: “The object was to give them the same purpose and most of them were escaping from their conventional and regimental discipline – they didn’t fully appreciate they were running into a much more exacting type of discipline...We had to get down to training immediately.”

The documentary, presented by journalist and author Ben Macintyre, shows footage of one unique method of training which the unit developed – jumping off the back of trucks at speeds of up to 40mph to practice landings.

The SAS grew into a military unit able to launch daring and devastating raids on German lines.

But the first mission in Libya to parachute into enemy airfields and blow up their aircraft ended in disaster when the worst storm in 30 years hit the area.

The mission went ahead despite the appalling weather – but only 21 of the 55 men involved returned. Some were captured, some were killed in an aircraft which was shot down and some were unable to release their parachutes on landing in the fierce winds and were scraped to death on the desert floor.

Stirling waited at a designated rendezvous point for two days in the hope more of his men might have survived.

Speaking about the incident he said: “It was tragic because there was so much talent in those we lost. We had to try and survive.”

*SAS: Rogue Warriors will be shown on BBC2 at 9pm on Monday 6th February