On a warm March evening in a medium-security prison in the south of England, two inmates were calmly chatting to a third man, who was rumoured to be a police informant. They walked with him to a cell, maintaining an air of civility until the door was firmly shut, then pulled out sharpened pieces of metal and stabbed him all over his body. The victim fell to the floor in a pool of blood and the two assailants quietly slipped away onto the landing to dispose of their weapons.

Incidents like this are becoming increasingly commonplace in Britain's prisons, partly due to a dramatic rise in gang activity. A recent report by the Independent Monitoring Boards prison watchdog found that assaults committed by inmates are doubling in many jails; in fact, the number has increased by 11 percent within the last year, and now stands at a record high.

So what's going on?

The stabbing of the suspected police informant was committed by members of Death Before Dishonour (DBD), a prison gang that formed at HMP Wakefield in the close supervision centre (CSC), a special unit reserved for inmates who are considered too much of a risk to staff and other prisoners to be allowed in the general population.

Predominantly white and involved in an ongoing feud with Muslim inmates, DBD is responsible for a stream of violent and disruptive incidents in the segregated prison areas of the institutions they operate in.

Yusuf, an inmate who was victimised by DBD members while in a CSC, tells me that almost every white prisoner who spent time in the unit ended up joining the gang. He claims to have been subjected to constant threats and intimidation, and believes that this was part of an organised attempt to get him transferred to another location so that he could be replaced by another gang member.

"Their strategy was to cause so much chaos and do so many disruptive things that pressure would be placed on the guards to get the non-gang-members on the unit moved elsewhere," he says.

Yusuf also claims that DBD has an "attack-on-sight" policy towards Muslims in lower-security prisons, aimed at getting members in these institutions transferred to the Category-A prisons where their fellow gang members are housed. He also mentions another gang aligned with the DBD, the "Piranhas", which started life in prisons in Liverpool and Manchester and has since spread throughout the country.

According to Yusuf, the Piranhas are equally intent on terrorising Islamic inmates, but don't limit themselves to acts committed within the prison system. He claims its members have previously paid teenagers on the outside to set fire to the houses of the relatives of prisoners who convert to Islam while in jail, but this was impossible to corroborate.

Although racial tension is nothing new in British jails, organised, ethnically-defined gangs with distinct names and identities are. Given that the UK lacks the long history of race-based prison gangs seen in the US, why are these groups now beginning to emerge in our jails?

Political scientist and crime researcher David Skarbek has studied prison gangs all over the world. He explains that a consequence of large prison populations is that, if groups are held accountable for the actions of an individual, as is sometimes the case in gang-infested institutions, inmates need easy ways to immediately recognise who belongs to which group – and race is a low-cost characteristic that performs this function.

With 81 of the 120 jails in England and Wales now officially classed as overcrowded, this theory might go some way towards explaining the growth of Islamophobic gangs like DBD and the Piranhas.

British prison gang expert Jane Wood believes that cuts to the number of prison officers is another factor, explaining that a lower number of staff has resulted in prisoners feeling emboldened to operate as gangs and commit illegal acts. She also believes that gang membership behind bars has grown over the last two decades, and that if it isn't adequately addressed the number of these gangs will most likely double within the next few years.

"I remember when I first started researching this, which is going back to 1999 or 2000, and it was really difficult to find any prisoners who would say that gang activity was going on in the prison or any member of prison staff who actually believed that it was happening," says Wood. "Now, it seems to have escalated... The lower numbers of staff have given prisoners more freedom to form groups and carry out prison gang activities."

Given that the new breed of prison gang appear to be somewhat different from those of old, is it time for changes to be made to the prison system's anti-gang measures? Can a set of procedures put in place when the gang situation was radically different be successfully applied today?

Dana is a serving guard at a jail with a substantial gang problem. She told me that the prison uses a gang matrix – a database of gangs and their enemies – to ensure that rival gang members aren't placed on the same wing as one another.

The problem is that this database only includes gangs formed on the streets, meaning those created behind bars – such as DBD and Piranhas – are unaccounted for. It also relies on inmates self-reporting gang-affiliations. Many inmates are wary of doing this as they feel that it will place them under additional scrutiny from guards. Cons also sometimes claim membership of a gang to avoid going on certain wings, even if they're actually unaffiliated, which further confuses matters.

When I asked what she thought could solve the gang problem, Dana sighed and told me it will never change; that it's just a fact of life.