IMAGINE your name has been put on an official database, without consent, notice or explanation. As a result, it may be harder to go to college or rent a flat. It may even increase the odds of being deported. That, according to a report by Amnesty International, a charity, may be the fate of thousands of young men in London.

The database in question is the “gang violence matrix”, maintained by the Metropolitan Police. Launched after riots in the capital in 2011, it keeps track of suspected and known gang members. The most recent data show that it included 3,806 names in October 2017, each with a score indicating the risk of the individual committing violence. To add someone to the database, police require two sources of information demonstrating links to a gang. The evidence can be thin: family ties, say. A third of those on the list have never been convicted of a serious crime.

The database informs police tactics, such as where to perform stops and searches. But information from it can also be shared with other authorities, like schools and council housing services. The idea is to improve co-ordination of anti-gang efforts. Yet Amnesty fears that this sharing of information may encroach on people’s lives. One man lost his college place when the college discovered he was listed as being involved in a gang. Being stopped and searched can become the norm. For some, it may feel “akin to living in a police state”, says Patrick Williams, a criminologist at Manchester Metropolitan University.

Amnesty notes the high proportion of ethnic minorities in the database. Black people make up 78% of its names. It is hard to know how well this reflects the make-up of London’s gangs, not least because the Met’s definition of a gang is not clear. But only 27% of the perpetrators of violent crime against young people in London are black. A gang database used by police in Manchester shows a similar skew towards minorities, according to a report in 2016 by Mr Williams and Becky Clarke, another Manchester criminologist.

Does the matrix reduce violence? The Met says it is a useful intelligence tool. A nationwide rise in violent crime in the past two years, and a surge in murders in London, have put gangs on the agenda. Yet the London mayor’s office found that in 2016 only 5% of the capital’s knife crime which resulted in an injury was linked to gangs.

There may be less intrusive ways for public services to share data. Richard Garside of the Centre for Crime and Justice Studies points to a programme in Cardiff, where hospital staff share anonymised data with the police about where victims were attacked. The model has cut crime while protecting privacy.

The mayor is reviewing the Met’s approach to gangs, as part of an inquiry into knife crime. Meanwhile the Information Commissioner’s Office, a watchdog, is considering whether the force is in breach of data-protection laws. If it is, that could mean more limited access to the matrix.