Tens of thousands of police officers and staff have not had the proper vetting clearance which could root out sexual predators.
Her Majesty's Inspectorate of Constabulary and Fire and Rescue Services (HMICFRS) estimates that around 35,000 of the 202,000 people working in forces across the country do not have the appropriate level of vetting.
On average 13% of people in each force have not been vetted, but in some areas the figures were much higher.
Inspectors believe 37% of the Metropolitan Police do not have the correct vetting, followed by nearly half of the second biggest force in the country, West Midlands (52%), followed by 42% in Thames Valley Police.
The HMICFRS report into those who abused their positions for sexual purposes interviewed victims as well as staff and officers.
Among the cases highlighted were rapist Ian Naude – who preyed on his 13-year-old victim while a student Cheshire police constable – and West Midlands officer Palvinder Singh, who bombarded vulnerable victims with hundreds of messages.
Inspector of constabulary Zoe Billingham said the figure of 35,000 for staff who had not been vetted properly was a "best estimate", but admitted the real figure could be higher.
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She said there "there have been cases in almost every single force" with victims including suspects, victims of crime,
children, and vulnerable people.
She added: "Most of the victims are women and most of the perpetrators are men.
"Too often their abuser plays the role of the saviour in policing. They play the role of the knight in shining armour."
She also said police predators who "get wind" of a complaint "run before they are caught and move from force to force", adding: "There is no agreed way of passing soft intelligence between forces at the moment of transfer."
Cheshire Constabulary PC Ian Naude, 30, was jailed in December 2018 for raping a 13-year-old girl.
At his trial he was described as a "committed paedophile" who only joined the force "to gain the keys to a sweetshop".
During the investigation it emerged he had been named as a suspect to Staffordshire Police in January 2017 for sexual activity with a child.
The same month, the West Mercia force also received an allegation he was persistently asking a girl on social media to meet him.