Karl Lokko, 26, left the gang scene in Brixton, south London, five years ago and now devotes his time to helping young people reform their lives.

Describing his time in the gangs, he said: "It got really hairy at one point, to the point that I used to leave my house with a bulletproof vest on every day. I still get pains today because of it.

"I was cut on my face at 14, stabbed in my chest the same year, I got butchered in my back at 18 and shot at several times."

But he began to see things differently when a fellow gang member's mother came to his aid.

He said: "Step by step my mind began to change and see that life is bigger than Brixton.

"When I realised that gangsterism isn't an acceptable way of living and that it was actually a deception, I had to tell everyone about the lie. I went everywhere and I said basically, 'Look, I love you but you've been lied to'.

"That was my full-time job - I did that seven days a week, 12 hours a day, literally in every kind of surrounding. It had a really big effect."

Mr Lokko became involved in Big Change - a foundation set up by Sir Richard's children Sam and Holly - and joined their first Strive Challenge two years ago.

It was through Big Change that the former gangster first saw a mountain and struck up a friendship with Sam Branson.

Mr Lokko said: "I was more suspicious of him than he was of me initially.

"I was a bit apprehensive and I had to work him out, but we went and did an Alpine training week in Chamonix. During that week we went up on a really sketchy ridge that couldn't have been no wider than a metre and 4,000 metres above sea level with death on either side.

"We were strapped to each other and a guide and just working out that process of getting from one side to the next, together, that collaboration, knowing that that is really collective destiny - I fall, he falls - it just hit home that we are the same.

"We have the same sort of anxieties, we had different strengths but we can pull them in together - we were just human and every label kind of just dissolved and since then it's still dissolved."

He said Big Change was like being part of a new gang - albeit a positive one - where the "tribalism" he experienced in Brixton could be felt again.

Mr Lokko added: "Since I denounced the street-level gang that I was in, I've been yearning for a gang that embodies the sort of ideals that I do and ultimately I've found that gang."

The Virgin Strive Challenge, which set off from Zermatt on Friday, hopes to raise £1.5 million for projects supported by Big Change.