More than half of the women employed at the London-based video game developer Rocksteady two years ago signed a letter to bosses accusing the studio of failing to prevent sexual harassment and inappropriate behaviour in the office, the Guardian can reveal.

The letter, dated November 2018 and signed by 10 of the company’s 16 female staff at the time, raised complaints about behaviour including “slurs regarding the transgendered community” and “discussing a woman in a derogatory or sexual manner with other colleagues”, and sexual harassment “in the form of unwanted advances, leering at parts of a woman’s body, and inappropriate comments in the office”.

Since then it is claimed that the response has amounted to one training seminar, and that multiple signatories have left the company owing to the lack of action.

One of the letter’s signatories, who asked to remain anonymous, said she had decided to share the letter with the Guardian because she felt people were still suffering from sexism, harassment and inappropriate behaviour at Rocksteady.

“I have heard everything from groping claims to incidents involving [senior staff], all of whom are men,” she said. “Yet the only thing we had as a result was a company-wide seminar that lasted an hour. Everyone who attended was asked to sign a statement confirming that they’d received the training. It felt that it was a just way for them to cover their arses.”

She said the letter was kept private in part due to the company’s devotion to secrecy, which is not unusual in the video game sector, and in part because staff members feared that if they left on bad terms they would be denied credit on the company’s upcoming game, an adaptation of DC Comics’ Suicide Squad.

The signatory said the dismissive attitude towards women had in the past carried over into the company’s output. “Rocksteady doesn’t have the best reputation for representing women,” she said, citing the highly sexualised design and costuming of characters such as Poison Ivy and Harley Quinn in previous games. “Sometimes you could see the surprise on their face when you said that’s not how women dress.”

Rocksteady acknowledged it had received complaints about sexual discrimination and harassment. “From day one at Rocksteady Studios, we set out to create a place where people are looked after, a place fundamentally built on respect and inclusion,” a spokesperson said.

“In 2018 we received a letter from some of our female employees expressing concerns they had at that time, and we immediately took firm measures to address the matters that were raised. Over the subsequent two years we have carefully listened to and learned from our employees, working to ensure every person on the team feels supported. In 2020 we are more passionate than ever to continue to develop our inclusive culture, and we are determined to stand up for all of our staff.”

Last Thursday, after the Guardian contacted Rocksteady for comment, management called an all-staff meeting where they discussed the letter for the first time. New initiatives were promised to prevent further discrimination, the Guardian understands.

The signatory cited recent changes at the French games firm Ubisoft as another reason for sharing the letter. Last month several executives, including the chief creative officer and second-in-command, Serge Hascoët, resigned amid allegations of misconduct and inappropriate behaviour at the company.

“I think a good outcome [of publicising the letter] is basically showing the games industry in general [that] no matter how big your company is, how much you promote it as supportive of diversity, if you keep putting your head in the sand you will eventually be outed,” the woman said.

“I’d say 97%-98% of the developers there are incredible people, and it’s so unfair that this will land on them because a few people weren’t managed properly.”

Rocksteady, which is owned by Warner Bros Interactive, is one of the leading lights in the UK’s video game development industry and counts the Batman Arkham series among its titles.

Video games now account for half of all UK entertainment revenue, with the industry contributing more than £4bn to the economy.