Thousands of inmates are being beaten, bullied and intimidated by prison gangs which are becoming increasing powerful and violent as the prison population soars, The Observer has found.

The gangs are involved in everything from drug trafficking and illegal gambling to assault and murder. Some have become so powerful that members make hundreds of pounds a week from the jail drug trade. This money is put aside for when the prisoners are released or used to provide better treatment inside. Those who fail to pay their debts risk violence against themselves or their families.