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ONE, the anti-poverty charity co-founded by Bono of U2 and Bobby Shriver of the Kennedy political dynasty, has apologized for what it described as several years of bullying at its South African office.

The charity issued the apology as The Mail on Sunday, a British tabloid, was preparing to report that employees at the charity's Johannesburg office claimed to have been "subjected to a 'toxic' culture of bullying and abuse."

ONE said it couldn't corroborate some of the claims in the article, including comments attributed to a married employee whom The Mail on Sunday quoted as saying she felt pressured to have sex with a member of the Tanzanian Parliament. But in a statement datedFriday, it said its own investigation had uncovered evidence of "unprofessional conduct" at the Johannesburg office from late 2011 to 2015.

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ONE charity co-founders Bobby Shriver, left, and Bono at an appearance in London in 2006. AP

"Staff were called names, and some said their manager put them to work on domestic tasks in her home," said Gayle Smith, the charity's chief executive. She described the behavior as "bullying and belittling of staff."

Smith said the investigation found that the charity's management failed to adequately address the behavior or to properly inform its board of directors, who include former British Prime Minister David Cameron; Sheryl Sandberg, chief operating officer of Facebook;