Nine out of 10 police officers from one force have not had a recent background check, a BBC investigation has claimed.
It has been reported around 14,000 officers from 17 forces in England, Wales and Northern Ireland have not been vetted in line with policy.
In Northumbria, almost 3,000 - or 90% - of officers had not been subjected to up-to-date checks, the broadcaster claimed citing requests made under the Freedom of Information act.
Her Majesty's Inspector of Constabulary Mike Cunningham told the BBC: "There is no doubt that forces who have not vetted or re-vetted large numbers of their staff are subject to a vulnerability."
The process of vetting looks at an officer's background, family and friends to check they are not associated in criminal activity.