From the BBC:
"The National Police Chiefs' Council (NPCC) defended the practice of paying informants - or covert human intelligence sources as they are officially known.
Deputy Chief Constable Roger Bannister, the council's lead on the issue, said: "The intelligence provided helps to prevent and solve the most serious of crimes and is vital in bringing offenders to justice through the courts."
But Neil Wood, who worked as an undercover police officer and ran many informants, told Radio 5 live the tactic has its limitations when it comes to drugs.
"Nobody wants to inform on the drug lords because of fears of violent reprisals, so it's only the low-lying fruit that gets caught out - and the trade continues regardless. Nobody can call that effective. Overall it does little to bring down the level of overall crime."