The Confederation of British Industry has admitted it hired “toxic” staff and failed to fire people who sexually harassed female colleagues.

Britain’s biggest business lobby group said the failure to act allowed workers to believe they would not face punishment for harassment or violence against women.

In an open letter responding to an independent investigation into two rape allegations at the CBI, president Brian McBride said bosses “made mistakes … that led to terrible consequences”.

The CBI’s future remains uncertain after the assault claims sparked an exodus of firms, leaving the group’s reputation as the voice of British business in doubt.

Earlier on Monday, chancellor Jeremy Hunt said there was “no point engaging” with the group “when their own members have deserted them in droves”.

The letter said that, effective immediately, it will “operate a zero-tolerance approach to sexual harassment and bullying behaviour”. It said a “number of people” have been dismissed for not maintaining the standards expected of them.

It follows the dismissal of former director general Tony Danker two weeks ago after he was accused of making unwanted contact with a woman who works for the organisation.

He said his reputation had been “destroyed” by the allegation and claimed he had been made a “fall guy” for a wider crisis. He told the BBC his name had been wrongly associated with separate claims, including the rapes which allegedly happened before he joined the CBI.

Mr McBride said the board and senior leaders felt “a collective sense of shame” for having let down staff. “In retrospect, we now know that we were complacent. And we made mistakes in how we organised the business that led to terrible consequences,” Mr McBride said.

It also follows articles in The Guardian, which spoke to more than a dozen women who claimed to be victims of sexual harassment while working at the CBI. Two of these women said they were raped.

Mr McBride said that the group had not managed to filter out “culturally toxic people” when hiring. The group also did not act properly when allegations were made internally against these people, he said.

“We tried to find resolution in sexual harassment cases when we should have removed those offenders from our business,” he wrote.

“In retrospect, this last point was our most grievous error, which led to a reluctance amongst women to formalise complaints.

“It allowed that very small minority of staff with regressive – and, in some cases, abhorrent – attitudes towards their female colleagues to feel more assured in their behaviour, and more confident of not being detected.

“And it led victims of harassment or violence to believe that their only option was to take their experiences to a newspaper.”

Mr McBride added that board members and senior leaders at the group had believed the CBI had a strong corporate culture, and that they have experienced difficult emotions since the disclosures were published.

“The greatest of these emotions is a collective sense of shame, for having so badly let down the enthusiastic, ambitious and passionate people who came to work at the CBI.

“They rightly expected to be able to do so in a safe environment, and we failed them.”

The president said the group would try to regain members’ trust.

“Whether that is possible, I simply don’t know. That is, of course, for each of you to decide,” he said.