Police in the UK are trialling a new “stop and scan” power, which lets them check the fingerprints of unknown individuals against national criminal and immigration databases.

Officers will be able to stop anyone when an offense is suspected and scan their fingerprints using a mobile device if the individual cannot otherwise identify themselves. The scanners will check fingerprints against 12 million biometric records held in two databases: IDENT1, which contains the fingerprints of people taken into custody, and IABS, which contains the fingerprints of foreign citizens, recorded when they enter the UK.

Speaking to Wired UK, project manager Clive Poulton, who is helping oversee the trials for the Home Office, said: “[Police] can now identify the person in front of them — whether they are known to them or not known to them, and then they can deal with them.”

The Home Office and police forces involved say stop and scan is simply a way to speed up checks that officers would otherwise have to make at police stations. But privacy and human rights advocates warn that the mobility of the technology and the lack of oversight in its deployment means it could foster abusive policing tactics.

“There’s a really good reason people have to take suspects to the station.”

Martha Spurrier, director of UK advocacy group Liberty, said the technology could exacerbate problems associated with current stop and search powers, which are disproportionately used to target minorities and are often cited as a cause of tension between police and local communities.

“The problem with these mobile applications is that there’s nothing to stop an individual officer acting on their worst prejudices,” Spurrier told The Verge. “With taking fingerprints or interviewing subjects, there’s a really good reason people have to take suspects to the station, because it [allows for oversight]. These are safeguards to make sure a police officer isn’t wandering around an estate, fingerprinting people at random.”

There is a legal framework in place for collecting this sort of information, the Police and Criminal Evidence Act, or PACE, which was passed in 1984. But, says Spurrier, PACE is outdated and has been amended over the years to keep up with new technology without proper debate, public or otherwise. “How can we say this is policing by consent when there is no parliamentary scrutiny?” she says.

Other worries about the new technology include how biometric data might be shared between different enforcement agencies. For example, police officers might collect fingerprints from individuals without proper cause and then hand this data over to the Home Office where it might be stored indefinitely without the individual’s knowledge.

The Home Office has long promised to publish a comprehensive overview of its biometric data policies, but this has been delayed since 2012. In the meantime, it’s been criticized repeatedly for holding onto data for too long, and failing to inform individuals of their rights.

The veracity of the fingerprint databases that underpin these scans was also called into question recently. In January, a parliamentary committee tasked with investigating government policy on immigration found a 10 percent error rate in the Home Office’s list of “disqualified people.” This, said the committee, has fed into a string of injustices, from individuals being denied bank accounts because of their supposed immigration status, to citizens being held in detention centers only to be later released without apology or explanation.

The new stop and scan power is currently being trialled in West Yorkshire, with 250 mobile scanners issued to officers. After that, reports Sky News, the system will be rolled out nationwide.