Inadequate vetting for thousands of police personnel
Even though criminal records and background checks have been stricter since 2006, many police forces have not adequately vetted officers and staff.
The National Police Chiefs' Council is aiming to cut levels of non-compliance.
Her Majesty's Inspectorate of Constabulary and Fire and Rescue said it was seeing too much corruption and abuse of power - sometimes for sexual gain.
The National Police Chiefs' Council said more cases like this were being reported because of the work they had done in highlighting the issue.
In a statement, it said: "There is a gap and we will work really hard to sort that out. We are confident the number of officers and staff who have not been retrospectively vetted before 2006 will fall in the coming months."
HMICFRS said the low levels of vetting uncovered by the BBC could affect public confidence in the police.
"We're currently inspecting forces to check that they've cleared their vetting backlogs," it said.