The law of Joint Enterprise allows the courts to impose a life sentence on people even if they took no direct part in an offence.
People can be convicted of serious offences, even murder, whether or not they wielded a knife or threw a punch.
The police say the law is a valuable weapon and deterrent and its use has helped them tackle gang culture and violence.
But campaigners see it as a way of locking up innocent people, some of whom just happen to be at the scene of a crime.
In the absence of any data, a committee of MPs is now trying to establish how widely the law is being used.
Perhaps most famously the law was used to convict Derek Bentley of the shooting of a police officer because he said to his accomplice Christopher Craig - who actually pulled the trigger - "Let him have it."
Joint enterprise was also used to convict three teenagers of the murder of Garry Newlove, whose assault by drunken youths in 2007 was seen as proof of David Cameron's broken Britain.
The coroner found Mr Newlove died from a single kick, but all three, including Jordan Cunliffe who was 15 at the time, are serving life for murder.
Garry Newlove's killers were convicted under joint enterprise
Mr Newlove's widow Helen has said his sentence is entirely justified:
"Life should mean life. If you are saying Jordan Cunliffe did not attack my husband, he certainly stood next to that body and watch them beating and beating. You're as guilty as a person, because you are watching that act, you are guilty of that act."
But Cunliffe's mother believes the joint enterprise law is being used to convict those only peripherally involved in a crime - or as she says, in her son's case - simply in the wrong place.
Commander Simon Foy of the Metropolitan Police has been responsible for taking the message of joint enterprise into London schools to warn youngsters what can happen if they get involved with the wrong crowd.
"I make no apologies for how we have used this as a principle to get across what we hope is a positive, constructive, educative message," he says. "The most important thing we can do in these events is to stop someone getting stabbed or killed."
Police are confident the law is helping to keep young people away from trouble and has even had an impact on levels of knife crime in London.
But there is a very different view of it in the dense estates where gang culture has flourished:
"From what I have seen and from cases I know with joint enterprise they do spend a lot of time building up a case but it seems to be easier to build a case of joint enterprise, it gets rid of the whole guilty before proven innocent thing," says Yohanes Scarlett, a student who grew up an estate in West London. "It's almost like you are guilty - now prove to us you are innocent. It's a lot harder to do that."
Some believe the alienation caused youths to take part in the summer riots
Yohanes says that young people there think they are being targeted:
"I think a lot of people have experienced JE either personally, or they have friends who have gone through it, so it starts making people not trust the police as much - they feel police are specifically targeting their communities."
Kumani, a local youth worker, says many young people are scared of being simply in the wrong place and falling foul of the law.
He says it has caused resentment towards the police and an alienation which he thinks partly explains why some people took part in the summer riots.
"I know young boys who have been hit with a joint enterprise charge and, having been in the criminal system at the age of 13 or 14 will come out at the age of 26 and have spent most of their adult life in prison."
Kumani introduces me to several young people who all believe that joint enterprise is responsible for sending the innocent to jail. And campaigners agree with them.
The campaign group Jengba says it is fighting 256 cases. Of those, 215 prisoners are serving life for murder.
Twenty-five percent were under 21 when convicted and 59% are from black and ethnic minority communities.
The police insist the law is applied in a considered way. While acknowledging that its use has increased, they have no data on how many people have been charged under it.
The Ministry of Justice and Crown Prosecution Service also keep no figures to show how many people are charged under joint enterprise, so one of the first tasks of the Justice Select Committee of MPs will be to establish how widely the law is being used.
Potentially more decisive will be a Supreme Court judgement due later this year in which joint enterprise and its use to take on gangs will be very much on trial.
Watch Liz Mackean's film on Joint Enterprise on Newsnight on Tuesday 1 November 2011, then afterwards on the BBC iPlayer and Newsnight website.
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