London gangs are drawing up and disseminating lists of teenage girls whom they consider to be legitimate rape targets, as sexual violence is increasingly used to spread fear and antagonise rival groups.

The so-called sket lists (sket is street slang for "sluts") have, according to youth workers, prompted attacks so brazen that girls have been dragged from school buses and sexually assaulted. Police and charities say they have recorded an increase in the use of sexual violence by gangs, including incidents of revenge rape, where the sisters and girlfriends of rival gang members are targeted. Claire Hubberstey, interim chief executive of Safer London Foundation, a charity working with young people to reduce crime, warns that gangs are using sexual violence in the same way that they use dangerous dogs to parade their masculinity.

Scotland Yard has confirmed that sexual violence against women by gangs is now "at the top of our agenda" following initiatives that have seen gun crime fall by 17% and knife crime offences by 11.5%. Det Supt Tim Champion of the Metropolitan police's Operation Trident gang crime command, said: "The first thing we had to do is stop people killing each other. The focus now clearly is on women. It's as prevalent as carrying a knife or a gun – raping a girl in a gang."

Hubberstey said gang members were taking advantage of low conviction rates for rape, viewing sexual violence as a less-risky means to inflict pain on rivals or spread fear than carrying a weapon. "Criminals are clever, they know if they are caught carrying weapons they face a lengthy sentence; it's risky carrying a gun. The use of sexual violence is the same sort of thing as having a dangerous dog; it creates fear, it's non-traceable, and they are also taking advantage of low rape conviction rates even when there are witnesses," she said.

The emergence of "sket lists" of young women considered eligible for attack is, say frontline youth workers, indicative of how gangs perceive that they can rape with near impunity. "They put the names of young women on a list and circulate through BBM [BlackBerry Messenger]. These women become active targets on the way home from school or wherever. They are legitimate targets to be raped and sexually assaulted if their name appears on a sket list," said Hubberstey.

Those appearing on sket lists were sometimes teenagers who had been secretly filmed engaging in a sexual act by a gang who then threatened to upload the footage on to social media if she refused further sexual demands. Hubberstey described such blackmail as fairly common, and Champion described it as an effective controlling mechanism.

Carlene Firmin, head of MsUnderstood Partnership, which aims to address youth sexual inequality, said that young women connected to gangs were viewed as "currency" by rival outfits and attacked accordingly, even if such acts occasionally did not have the impact they wanted. "Sometimes they [a gang] think it will have a really detrimental effect but actually the boys are not bothered, usually it's about punishing girls directly rather than boys by proxy.

"The punishment of young women occurred more often when they were perceived to have lied or, say, were arrested and the drugs they were holding for other people were confiscated," said Firmin. Hubberstey added: "They target cousins, girlfriends, siblings, we see a lot of that." Her charity had found some girls being offered to rival gangs to bolster allegiances or foster a truce. "They say, 'Here's some young women to do whatever you want,' we have lots of examples of that, really brazen activity and indicative of a lack of fear."

Figures from the Safer London Foundation reveal that more than 500 young women were victims of gang-related sexual violence in the past year, a figure Hubberstey describes as just the "tip of the iceberg".

Scotland Yard's latest intelligence identifies 3,495 gang members in 224 gangs in London, although just 40 were found to be female.

The Metropolitan police is currently mapping the problem in an attempt to ascertain the scale of the issue and identify "hotspots" of abuse. Firmin said they were identifying girls as young as 11, a vulnerable age due to the transition from primary to secondary school and the pressure to fit in and make friends, as potentially vulnerable.