In 1999 I witnessed a gang rape in Sierra Leone. I was forced to watch a group of rebel soldiers taking it in turns to rape a young girl in front of an audience of jeering men. It was the height of the civil conflict and rape had become a devastating weapon of war. When I moved to Britain I believed I had escaped such horrific sexual violence. As my Dispatches investigation tomorrow night shows, I was mistaken. Gang rape is happening here – and what I have found most disturbing as an African is that a disproportionate number of these attacks are being carried out by black or mixed-race young men.

Towards the end of last year, police and child welfare experts working on Channel 4's Street Weapons Commission told us of their concerns about gang rape. Then two big cases hit the headlines.

In December, nine schoolboys, some as young as 13 at the time of the attack, were convicted of raping a 14-year-old girl. She was dragged between tower blocks in Hackney where she was threatened with a knife, hit and raped during an ordeal that lasted an hour and a half – some of which was filmed on mobile phones. In January, three men were convicted of gang raping a 16-year-old with learning disabilities for two hours before dousing her with caustic soda in an effort to get rid of the evidence.

Download the new Independent Premium app Sharing the full story, not just the headlines

How prevalent is this crime and why it is happening in Britain? Despite the seriousness of the crime, I was amazed to discover that no national statistics exist: gang rape is simply not recorded as a separate crime category. So over a period of several months we set about collating our own.

We approached the Crown Prosecution Service, the Association of Chief Police Officers, all 50 police forces, crown courts, barristers and rape referral centres to try to establish the numbers.

One of the few police forces to have begun recording the figures of reported gang rape is the Metropolitan Police. In 2008 alone, they received reports of 85 gang rapes. Using the Met's definition of gang rape – those involving three or more perpetrators – we began to look at the number of convictions. We tracked down 29 cases, from January 2006 to March 2009, in which a total of 92 young people were convicted of involvement in gang rape.

One fact stood out. Of those convicted, 66 were black or mixed race, 13 were white and the remainder were from other countries including Afghanistan, Iraq and Libya.

Clearly this is not a crime exclusive to black communities, but I found it impossible to ignore the fact that such a high proportion were committed by black and mixed-race young men. As a black man as well as a journalist, I wanted to understand what lay behind such attacks. So I spoke to victims, groups of black and mixed-race teenagers, youth and social workers and community leaders.

The groups of young men I met in London expressed some profoundly disturbing attitudes towards girls and sex. The boys explained how they make arrangements for "line-ups" in which one girl has oral sex with up to six or seven of them at one time. These arrangements might be made at school or on mobile phones.

Sometimes these girls initially consent because they want to be popular. But these events can spiral into rape because the boys consider that any girl who is prepared to agree to a line-up can be considered fair game. One boy told me: "If she wants to go and meet a bag of boys then she's probably a jezzie [slut], and if she's going to a house it's over – she's going to get beaten [have sex]."

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

In other instances, as some of the victims in our film describe, girls can unwittingly walk into a trap, innocently visiting someone's house to listen to music or watch a film only to discover that a group of boys are lying in wait. Or they might be hanging out with friends in a park and suddenly realise they have become surrounded by a group of boys intent on sex.

For both boys and girls, the line between this sort of group sex and rape seems to be blurred. A girl might agree to have oral sex with two or three boys but then be ordered to have sex with six or seven. The teenage girls I met told me that boys simply don't understand what rape is. And yet this is a crime that can ruin lives and is punishable by life imprisonment.

Occasionally gang rape is used to punish a girl for minor transgressions against gang members. In one of my most shocking interviews, I met a girl who admitted she had helped to set up girls for gang rape. As the girlfriend of a gang member, she organised these rapes, partly out of fear and partly to fit in.

She admitted she was terrified of being raped herself and had walked away when witnessing a girl being gang-raped at a party because she feared she might be next: "There was just loads of boys and the girl's tights were ripped up, like, she was bleeding as well, because I think she was a virgin, and they were just taking turns on her basically, and she was crying, and I didn't get involved because I thought if I get involved they're gonna turn on me."

The victims' descriptions of their attacks are horrific. One young victim likened her attack to being "pulled and pushed around like a rag doll", while another 14-year-old girl described her ordeal when she was gang raped by a total of nine boys who told her that she was not the only girl they had attacked. In that case, nine boys were subsequently convicted of raping her. The youngest perpetrator was just 12 years old.

I found there was concern among black communities about this violence. The Rev Joyce Daley, from the Black Parents Forum in Hackney, told me that gang rape is not a rare or one-off phenomenon. It is happening on a regular basis. She said: "It could actually explode on our very streets." Steve Griffith, a youth worker in King's Cross, said: "I see too much abuse of young women on the streets."

Gang rape, while constituting only a tiny percentage of all rapes in the UK, is a horrible reality in this country. The nature of the crime is so appalling that much more research needs to be carried out into its causes. But what seems evident from my investigation is that the key to preventing it will be changing the way young men view women and the kind of group sexual activity they are engaging in at such a young age.

Sheldon Thomas, a youth worker in Brixton, said: "We've got a generation that looks at sex as if it's nothing, and treats disrespecting women as if it's nothing. These guys are like 13, 14 and 15, and their actual attitudes towards young girls – towards sex – is mind-blowing. It's actually leaving you asking: where are their morals, where are their values?"

Sorious Samura presents 'Dispatches: Rape in the City' on Channel 4 tomorrow night at 8pm