RECRUITING informants is a legitimate, if controversial, law enforcement tactic that is as old as policing itself.

In the UK, the individuals who agree to provide information to officers are known as “covert human intelligence sources” – Chis –and strict guidance covers the practice.

According to Police Scotland’s standard operating procedure, a detective superintendent fulfills the statutory role of “authorising officer” for Chis and approves their use.

Separately, a “controller” must at least hold the rank of inspector and is responsible for maintaining legal and ethical standards in Chis operations.

At a lower level, a “handler” will be an officer who has the day-to-day responsibility for maintaining a formal relationship on behalf of Police Scotland with the Chis. He or she would also assess the quality of any information provided.

Police Scotland spent almost £400,000 on Chis between 2013 and 2015, amounting to nearly £16,000 a month for the force’s first two years.