BBCOutside the security barrier at HMP Belmarsh in south-east London, a man pulls up his black T-shirt to show us his scars.
“I’ve been stabbed by gangs – look at this,” he says. “And these are gunshot wounds, also from gangs. Man, it’s crazy – I could have died.”
A thick line marks a healed cut from just under the man’s neck all the way down to his navel. On his chest, the bullet wounds are circular and protruding.
He won’t give us his name or explain exactly what happened. He was in a gang when he was in prison, he says. And although he didn’t receive these injuries while inside, they were the work of a prison gang – operating on the outside.
“If I talk to you, I’m in trouble – so I can’t,” he says. He appears to be in his 20s or early 30s, and speaks quietly. “It’s brutal in jail. You’re either in or out with the gangs – and it’s better to be in, or you’re done for when you get out.”
BBC News spent two days outside London's HMP Belmarsh and HMP Isis speaking to people visiting and others being released to gauge how gangs in jails are operating.
Overcrowding is making it increasingly difficult to keep rival groups apart in prisons. In September, the government released more than 1,700 offenders early, in a controversial scheme aimed at easing the overwhelming pressure on the prison system.