3 March 2015 Last updated at 01:18 GMT
Aimen Dean is a founder member of al-Qaeda, who changed tack in 1998 and became a spy for Britain's security and intelligence services, MI5 and MI6. Interviewed by Peter Marshall, he describes his years working in Afghanistan and London as one of the West's most valuable assets in the fight against militant Islam.
Dean was brought up in Saudi Arabia, where opposition to the Soviet occupation of Afghanistan in the 1980s made military jihad a noble concept. He was a teenager when Yugoslavia splintered, and Bosnian Muslims found themselves in mortal danger from Serb nationalists. He and a friend, Khalid al-Hajj - later to become the leader of al-Qaeda in Saudi Arabia - set off to become mujahideen.
I would say it was the most eye-opening experience I ever had. I was a bookish nerd from Saudi Arabia just weeks ago and then suddenly I find myself prancing up on the mountains of Bosnia holding an AK-47 feeling a sense of immense empowerment - and the feeling that I was participating in writing history rather than just watching history on the side.