Bono has come forward to apologise after an internal investigation revealed a pattern of abuse and misconduct in the advocacy organisation he co-founded.

The One Campaign, a non-profit launched in 2004, focuses on fighting poverty and preventable diseases, particularly in Africa. "Actions. Speak. Louder." is its official motto.

In a letter to members posted on the organisation's website on Friday, CEO Gayle Smith said that between late 2011 and early 2015, staffers in South Africa were bullied, berated and belittled by managers. Employees told investigators that a supervisor made them work as party hostesses at her home on weekends.

The letter also says a female employee alleged that a supervisor made sexist and suggestive comments about her to a government official, and that she was demoted after refusing to have sex with the dignitary.

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Smith, who joined the organisation in March 2017, said One Campaign learned about the allegations after the employees, who had all since left, shared their grievances on social media in November of last year. Smith wrote that the organisation then launched an internal investigation.

The sexual harassment allegations are so far uncorroborated, Smith wrote, but she stressed that the organisation does not "discount any allegation".

Bono said he was "furious" after the allegations surfaced in November, as "the poorest people in the poorest places being bullied by their circumstance" was the reason One was set up in the first place.

"We are all deeply sorry," Bono told the Mail on Sunday newspaper. "I hate bullying, can't stand it."

Investigators were able to substantiate claims of bullying and harassment that included instances of the Johannesburg manager calling her staff names including "idiot", "stupid" and "worthless".

The report also found that executive managers were made aware of the abuse, through emails and complaints filed through human resources, and failed to stop it.

"The overall evidence from our investigation was sufficient for me to conclude that we needed to own an institutional failure and ensure that our organisation has in place the systems, policies and practices needed so that this never happens again,'' Smith wrote.

Smith said that both the employees who alleged the abuse and all leadership and executive management involved are no longer with the organisation.

The letter comes on the heels of reports of misconduct within other UK-based nonprofit organisations. Anti-poverty charity Oxfam recently came under fire over allegations that staffers working in Haiti following a devastating 2010 earthquake hired prostitutes.

The deputy executive director of UNICEF, a children's aid foundation, recently resigned after he was accused of acting inappropriately toward female staff while working at a different charity, Save the Children.

More than 1000 women recently signed an open letter calling for increased accountability surrounding sexual harassment and assault in the aid sector, with the hashtags #ReformAid and #AidToo.

Smith wrote that she has implemented new systems to help ensure accountability, including hiring a new executive director for Africa and planning to place a human resources manager on site in South Africa.

"We've got an obligation to find out how we do better,'' Smith said.