A London policeman who attempted to blackmail sex offenders and drug dealers has been jailed for six years.

PC Amerdeep Singh Johal, 29, was arrested by anti-corruption cops from Scotland Yard in July 2007. Johal was employed in checking names and address on the police database, called Crimint, on behalf of beat cops.

He abused the role to contact 11 convicted offenders and threaten to spill the beans on their crimes unless he was given "hush money". Johal requested between £29,000 and £31,000 for his silence, threatening to tell work colleagues or neighbours of convicted sex offenders about their crimes. In one instance Johal demanded £89,000 as a "goodwill gesture".

He also used Crimint to research drug dealers, including a member of a group rejoicing in the title The League of Four English Gentlemen.

The extortion racket unravelled after one of Johal's victims reported him to the police. Scotland Yard used an undercover officer to pose as a blackmail victim and offer to pay him £5,000.

Johal, of Ilford, texted a friend called Chatham Charlie looking for tips in how to invest the money. Johal's lawyers argued that their client was himself a victim of two extortionists, acting for a kidnapper called Baljit Bhandal.

Police failed to unearth independent sightings of the pair or the Bentley they supposedly drove, nor any supporting evidence. Nor, did Johal even tell his wife about the pressure he was supposedly under.

These arguments failed to convince the jury, who turned in a guilty verdict against Johal. An Old Bailey clerk confirmed Johal had been sentenced to six years' imprisonment for blackmail. He was also ordered to pay £32,000 in compensation.

The case has raised wider concerns about the misuse of police databases, which the Metropolitan police is keen to downplay.

A Scotland Yard spokesman told the BBC: "There are strict guidelines in place regarding the use of intelligence databases and if anyone abuses it that is taken extremely seriously." ®