Hundreds of police officers have been accused of abusing their power to sexually exploit people, including vulnerable victims of crime, a damning report has revealed.
Watchdogs laid bare the scale of the issue they described as the most serious form of corruption - and rebuked forces over their efforts to root out cases.
Her Majesty's Inspectorate of Constabulary (HMIC) was asked by then home secretary Theresa May earlier this year to investigate the extent of the problem.
HMIC's initial findings identified more than 400 claims of abuse of authority for sexual gain over two years.
But 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: "It is a matter of profound concern that any police officer should abuse their authority for sexual gain.
"The misconduct discovered in this report is shocking - it undermines justice and public confidence and there is no place in the police for anyone guilty of this sort of abuse."
Data collected by the watchdog 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, comprising 306 officers, 20 PCSOs and eight police staff.
All but one constabulary had at least one case, while 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.
While forces acknowledge the seriousness of the problem, some are still failing to recognise it as a serious form of corruption, the report warned.
Less than half (48%) of the 436 reported allegations had been referred to the Independent Police Complaints Commission.
Figures also suggested there was an "apparent disconnect" between the numbers of alleged cases and sackings as a result.
Some counter-corruption units did not have the capability or capacity to seek intelligence on potential abuse of authority for sexual gain, the report warned.
It also found that almost half of forces inspected were unable to audit or monitor use of all IT systems - limiting how much information could be gathered to spot officers or staff who may be accessing databases to identify vulnerable victims.
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.
She said: "While progress has been made, particularly in preventative and educational work, we are disappointed that a significant number of serious cases are still not being referred."
Mark Castle, chief executive of the charity Victim Support, described the allegations as "deeply concerning".
Elsewhere, HMIC raised concerns about levels of compliance with a national policy for vetting officers and staff.
Backlogs have built up, meaning many forces are failing to conduct regular checks on individuals after changes in their personal circumstances, or when they move from one post to another.
Overall, the watchdog said its police "legitimacy" assessment was positive, with high satisfaction among victims at how they are treated.
Two forces were graded as outstanding, 36 as good and five as requiring improvement.
Stephen Watson, National Police Chiefs' Council lead for counter-corruption, said: "Abuse of powers for sexual gain is a betrayal of our core responsibility to protect people from harm.
"It is the most serious form of corruption and it can never be justified or condoned.
"In recent years, we have focused on encouraging reporting and pursuing offenders.
"We now need to do more to continue to root out the disease and inoculate policing for the future.
"We are in the process of developing a national strategy to raise the standards of all forces in preventing this form of abuse."