The official police watchdog called on forces yesterday to do more to root out corrupt officers "seriously undermining" public confidence in the service.
The independent Police Complaints Authority told chief constables to consider using members of the security services or customs investigators to help hunt corrupt officers.
Tony Williams, an authority member, criticised forces that had failed to act promptly on evidence of alleged corruption by officers, allowing records to be "destroyed" and memories to "grow dim".
He said: "Most significantly, corrupt relationships had either ceased to exist or had become intermittent, thus reducing the scope for the successful application of intelligence techniques and the likelihoodof obtaining the requisite evidence."
Corruption inquiries are examining several forces, including the Metropolitan, Cleveland and North Wales ones. Mr Williams said: "There is no doubting the corrosive effect of police corruption. It breeds injustice, seriously undermines public confidence in the police service and saps the morale of honest officers."
He told a public policy seminar in London that corruption allegations could not be dismissed simply because they were based on the word of a criminal. "Witnesses from the criminal fraternity will often be derided by officers under investigation as unreliable even those who, ironically, are registered informants classified as 'usually reliable'.
"Drug dealers, addicts, thieves and prostitutes are well capable of telling the truth, and of being victimised."
The authority is concerned that forces do not give enough protection to staff who detect corruption among colleagues.
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