Hundreds of police officers have been accused of abusing their power to sexually exploit people, including vulnerable victims of crime.
The damning report described the widespread nature of what they said was the "most serious form of corruption" and condemned forces for not taking more action to root out cases.
Her Majesty's Inspectorate of Constabulary (HMIC) identified 436 reported allegations of abuse of authority for sexual gain received by forces in England and Wales in the two years to the end of March.
The allegations covered a total of 334 police personnel - 306 officers, 20 PCSOs and eight police staff.
More than a third (39%) of the allegations involved victims of domestic abuse. Arrested suspects and people with drug or alcohol problems were also thought to be among those allegedly exploited.
The watchdog said it was "likely" the problem was far worse.
HM Inspector Mike Cunningham said: "It is at least possible, probably likely, that the problem is more serious than the numbers that have been reported back to us.
"It's the most serious form of corruption. It is an exploitation of power where the guardian becomes an abuser.
"What can be worse than a guardian abusing the trust and confidence of an abused person? There can be no greater violation of public trust."
Home Secretary Amber Rudd said the findings were "shocking."
"It undermines justice and public confidence and there is no place in the police for anyone guilty of this sort of abuse," she said.
Despite the serious nature of the allegations, less than half (48%) of the 436 reported allegations had been referred to the Independent Police Complaints Commission.
Mr Cunningham said: "Forces need to become far more proactive in rooting out this most serious form of corruption, rather than only dealing with it once it has been reported."
He said the majority of officers and staff carry out their work with integrity and honesty - and are "appalled" at the corruption of their colleagues.
In the wake of the report, IPCC chair Dame Anne Owers has written to all chief constables in England and Wales urging them to ensure all cases regarding abuse of authority for sexual gain are referred.