She has responded to an investigation by The Times which found around one-quarter of the most serious police misconduct cases are being held in private despite a legal requirement for open hearings.
It found up to a quarter of misconduct hearings involving the most serious allegations of wrongdoing - including sexual impropriety, racism and homophobia, are closed to the public.
Among the cases in which anonymity has been granted are ones in which a senior officer has sex with a colleague on police premises, an officer who slept with a drug dealer and one who took money from a dead person.
The Times found that, of more than 40 misconduct outcome notices published relating to officers and staff in England and Wales in the past month, almost half were anonymised.
The newspaper also reported that figures obtained under Freedom of Information showed there have been 1,147 hearings since 2018.
The Times said forces were unable to say whether 502 of them were held in public or private, and of the remaining 645 hearings, one in four were held in private.
Forces routinely anonymise the names of disgraced officers and details of their offending, while public notices outlining the misconduct of sacked officers are being deleted from the public record within weeks in some cases.
That includes the online notice about the sacking of Wayne Couzens by the Metropolitan Police.
Police forces are required to publish the outcomes of hearings for at least 28 days with most deleted after that deadline. A source told the Times: “Forces are adhering to the letter of the law but not the spirit of the law. Accused police officers involve mental health concerns knowing it will result in their case being held in closed session.”
Responding to the findings, Mrs May said it was “immensely disappointing” some forces appeared unwilling to be properly scrutinised.
She said as Home Secretary she had introduced the principle that misconduct hearings should be held in public to improve transparency.
“While the vast majority of police officers serve with honesty and integrity, there are those who don’t operate to the same high standards,” she said.
“Opening up the disciplinary process gives the public confidence that the system is fair and that corrupt officers, or those guilty of misconduct, will be held to account.
"This shouldn’t have been a controversial step, which is why it is immensely disappointing to learn that more than six years on a number of police forces appear unwilling to open themselves up to scrutiny.
“According to the results of this investigation, too many hearings are still being held in private and the process of notifying the public of the results of those hearings is still worryingly opaque. It leaves the impression that the police, whose job it is to protect the public, are prioritising the reputation of the institution over the delivery of justice.”
She said the issue may be considered as part of the Sarah Everard inquiry, which will look at both the specifics of Wayne Couzens’ employment and wider policing issues.
“It would be wrong to pre-empt their work but I would imagine transparency, or the perceived lack of it, to be one of the issues they address,” Mrs May said.
“Surely now it is time for the police to properly ensure that where instances of corruption and misconduct do occur, they are rooted out with vigour on every occasion — and that this is done openly for all to see.”
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