U2 singer Bono has apologised after claims of bullying and abuse emerged at One, the charity he co-founded.
Among the allegations are that one female worker was demoted after refusing to "become intimate" with a foreign government official.
The woman's manager reportedly made "sexist and suggestive comments" about her to the official.
Bono told the Mail on Sunday newspaper: "We are all deeply sorry. I hate bullying, can't stand it."
The newspaper said the charity publicly admitted the issues hours after it had sent a long list of allegations at the end of an investigation, in order to allow One to comment.
The statement from the charity's CEO, Gayle Smith, said former employees had notified the organisation earlier this week about pending legal action over their complaints.
It said "historical issues" of mistreatment emerged in November when some former employees from its Johannesburg office told their stories on social media.
"The investigation yielded evidence of unprofessional conduct and, in particular, what I would characterize as bullying and belittling of staff between late 2011 and 2015 in our Johannesburg office," Ms Smith said.
"Staff were called names, and some said their manager put them to work on domestic tasks in her home," she added.
The allegation involving the woman who refused to "become intimate" with the foreign official could not be corroborated, the statement said - but Ms Smith said it was not being discounted.
In addition, the company said it had discovered that its African wing was not paying taxes between 2010 and 2015, but was instead a "non-resident taxpayer".
Ms Smith was not CEO at the time of the alleged offences.
She said the investigation showed "institutional failure" and that she had apologised to the former employees who agreed to speak to her.
Bono, meanwhile, spoke with the Mail on Sunday about its investigation, saying he was left "reeling and furious" when he learned of the allegations in November.
"My team and I heard concerns about low morale and poor management in this office but nothing along the lines of what emerged recently," he said.
"The head office failed to protect those employees and I need to take some responsibility for that," he told the Mail - adding that he would like to meet the victims to apologise in person.
Other prominent members of the charity's board include former UK Prime Minister David Cameron and Sheryl Sandberg, Facebook's chief operating officer.
The charity was founded in 2004 and says its aim is to tackle "extreme poverty". Its sister organisation, Red, was founded two years later, and raises funds through the sale of branded luxury goods, including those of Apple computers.