When BBC journalists began investigating the treatment of staff at the Welsh assembly, they were expecting to find accusations of bullying and mistreatment by politicians. Unexpectedly they uncovered claims of sexual assault against the BBC’s own staff.

The corporation sent an anonymous survey to 247 employees who work for assembly members in April. Thirty people responded, with three saying they had been sexually assaulted in their workplace.

One said they had been “inappropriately touched” by a BBC employee, while another said they had been sexually assaulted by a BBC worker. A third person reported sexual assault by an assembly member.

The survey submissions were all anonymous, meaning it is difficult to check the claims. The survey covered staff who work in the assembly’s Cardiff Bay headquarters and also in constituency offices.

The BBC will broadcast the allegations on Wednesday night in an edition of the BBC Wales Live programme. It will include the testimony of a further six employees of assembly members who said they had experienced workplace bullying or sexual harassment. One of these individuals used the survey to complain they had been “accosted aggressively by a BBC journalist” while doing their job.

Rhodri Talfan Davies, the director of BBC Wales, emailed staff before the broadcast and urged them to come forward with any concerns.

He said: “We obviously take these matters very seriously and – although anonymous allegations of this nature are extremely difficult to verify – we will of course look carefully at the information that has been broadcast.

“In the meantime, if anyone has any concerns they would like to raise confidentially, we wanted to remind you about the support that is in place.

“It’s so important that we have a positive culture here – one where you’re confident you’ll be heard, listened to and supported in dealing with any serious issues.”

Not all the accusations by assembly members’ staff were about the BBC. There was also an anonymous complaint about being “shouted or sworn” at by an assembly member and another complained about having to endure an “awful toxic environment”.

A BBC spokesperson said: “If anyone has any information they think could assist us in these matters, they can access our details on the BBC Whistleblowing website – which provides options to contact the BBC directly or via an independent organisation, Expolink.

“We are also reminding staff of the BBC’s policies should they wish to raise any concerns confidentially.”