Six serving police officers and a former officer face a criminal investigation into a WhatsApp group filled with "discriminatory, derogatory or pornographic" material, a watchdog has said.
All seven are under investigation for potentially breaching section 127 of the Communications Act by sending grossly offensive messages.
They have also been advised they are under investigation for potential gross misconduct, although it does not mean criminal charges or misconduct proceedings will follow.
The Independent Office for Police Conduct (IOPC) launched its probe following a referral from the Civil Nuclear Constabulary (CNC).
The CNC is an armed police force in charge of protecting civil nuclear sites and nuclear materials in England and Scotland.
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Another referal was made by a police force in the South West of England, which the IOPC did not identify.
Five of the officers work for the CNC, as did the ex-officer. The other officer has since moved to another force in the South West.
IOPC regional director Catherine Bates said: "The allegations against these officers and a former officer are extremely serious and concerning.
"On conclusion of our investigation we will decide whether the matters should be referred to the Crown Prosecution Service, whose role it is to determine whether criminal charges will follow.
"As this work is ongoing, it would be inappropriate to comment further at this time."
The IOPC said it cannot provide further information on the probe, including the name of the other police force, as it may identify those involved.
The investifation was launched after information came to light in a separate police investigation.
The case is not linked to previous probes into WhatsApp groups which had members including Sarah Everard’s killer, Metropolitan Police officer Wayne Couzens.
It is also not related to the arrest of an ex-Metropolitan Police officer suspended from his job as a Home Office immigration official over allegations he'd posted racist messages on the app.
The man in his 60s was arrested on Thursday on suspicion of offences under the Communications Act and misconduct in a public office and has been released on bail, the Met said.
This followed a BBC investigation which claimed a Government immigration official, who used to work for Scotland Yard, had been posting racist messages on WhatsApp.
It comes as Met Commissioner Sir Mark Rowley announced plans for a new anti-corruption unit to crack down on officers and staff who are misogynistic, homophobic or sexist, and other forms of abusive behaviour.
The Met’s new Anti-Corruption and Abuse Command has been launched to root out “criminal colleagues”.
More detectives will be hired and new technology will be used to overhaul work to track down corrupt officers, the force said.
Sir Mark said: “As part of my commitment to renew policing by consent and to bring more trust, less crime and high standards to the Met, I’m investing in extra officers and staff, skills and technology to root out those corrupt or abusive officers or staff.
“We will be proactive and unrelenting in actively seeking out these criminal colleagues. I make no apology for this ruthlessness in evicting those who undermine our integrity as we seek to reform the Met and give the public the confidence in us they deserve.”