The Metropolitan police shelved plans to reform its unit dedicated to protecting politicians and diplomats because of “resistant and uncooperative” officers, according to a former superintendent who was the force’s most senior woman of colour.

Nusrit Mehtab, who resigned in January last year, said the parliamentary and diplomatic protection command (PADP), where Sarah Everard’s killer, Wayne Couzens, once served, was “very male-dominated” and ripe for reform. The Met had had a chance to “put things right and they didn’t”, she added.

Mehtab is suing the Met alleging that a hostile and racist work environment forced her to leave. She claims the force harbours a culture that “enables and endorses people like [Wayne] Couzens to flourish”.

On Friday the Met announced that Louise Casey would lead a wholesale review of the Met in an effort to rebuild public trust, which will include a specific “in-depth, searching and rigorous review focused on PADP”.

Mehtab, who in a 30-year career at Scotland Yard worked in units including counter-terrorism, said her attempts at changing the PADP had been stymied.

“Only a few years ago I was going to be involved in bringing change to that command, but the officers were so resistant and so uncooperative that that reform couldn’t go ahead at that time,” she said. “The commissioner hasn’t had the appetite for cultural change and that’s why we’re here.”

According to a “senior Home Office source” quoted in the Sunday Times, the home secretary, Priti Patel, has grown frustrated with the Met’s “culture of defensiveness” since Everard’s murder.

Mehtab said much of the criticism of the Met this year described the “very things that I’ve been saying and the Black and Asian communities have been saying for decades, but we weren’t believed and we were vilified”.

She said the commissioner, Cressida Dick, had “created a culture of institutional denial” and should go. Mehtab also said the separate public inquiry announced by Patel last Tuesday should look at specialist units including the PADP.

“Some of the officers on these units have been on there for decades. And then [senior management] will say ‘you know what, they’re so specialised that we need to keep them there’. So they’ve become very entrenched in their ways and actually they move around that unit and they get promoted in-house. So there isn’t that scope to bring in new thinking.”

A security review of the PADP took place after the Westminster terror attack in 2017 when five people were killed, including PC Keith Palmer. A coroner ruled that the officer’s death could have been prevented were it not for “shortcomings in the security system”.

Mehtab said the internal Met review and the inquiry should scrutinise the “invisible canteen culture” of WhatsApp groups such as the one including Couzens that allegedly shared misogynistic and racist messages. She also said social media accounts, such as the network of Facebook and Twitter accounts held by anonymous retired or serving officers, should be looked at.

Citing the Macpherson report published in 1999, which identified a canteen culture that contributed towards the Met’s institutional racism, Mehtab said “racism and sexism has never gone away”.

“That culture has not been eliminated, it’s just been eroded, and it’s developed into a different form. It’s not the physical canteen any more, but an online one. It’s horrendous what they get away with, including abusing members of the public,” she said.

“Recruits are warned about WhatsApp messages when they first join and given a talk on professional standards.” However, she said she was not surprised to learn that Couzens was in a WhatsApp group with other officers who allegedly shared inappropriate material, referencing the Met police officers who allegedly took photos of the dead bodies of Nicole Smallman, 27, and Bibaa Henry, 46, and shared them on a private WhatsApp group.

An officer removed from duty guarding search cordons during Everard’s murder investigation is alleged to have sent an offensive graphic to colleagues on a WhatsApp group.

“There’s a culture that enables and endorses these people. It is a minority of officers, for sure, but they are protected by the majority because the Met creates an environment where they are enabled and endorsed and people like Couzens can flourish,” Mehtab said.

Since details emerged of how Couzens used his warrant card and police-issued handcuffs to get Everard into a car before raping and killing her, some anonymous social media accounts purporting to be serving police officers have defended the force’s reputation against what they feel is a media onslaught. Mehtab said these accounts had left some of her female former colleagues “horrified”.

“Don’t forget, they’re women who have to work with these men. The processes and systems in place to report misogynistic behaviour are not fit for purpose – they work on paper, but in reality female officers are not believed or supported.”

A spokesperson for the Met said the force had “done more than almost any other organisation to ensure that racism isn’t tolerated”. They said it was not the same force as it was 20-25 years ago, although “like all big organisations we know we can’t be complacent”.

A Met statement said: “The commissioner recognises that the murder of Sarah Everard has severely damaged public confidence … and that trust will be further diminished by the news of another officer charged with rape.”

It said policing was “complex and challenging”, and added: “Where we get it wrong we welcome scrutiny and where there are complaints we take these incredibly seriously and expect to be held to account for our actions, including through independent investigations by the Independent Office for Police Conduct. We have a clear set of values for staff and the code of ethics reinforces the standards of behaviour expected.”