U2 frontman Bono has apologised after claims were made that workers at a charity he co-founded were subjected to a culture of bullying and abuse.

The Irish singer, 57, said he was furious after the allegations surfaced last November.

He admitted the ONE organisation failed to protect employees at its Johannesburg office in South Africa.

The Mail today detailed a string of incidents, including claims from a woman who said she was demoted after refusing to have sex with a Tanzanian MP.

Bono said he was "deeply sorry" amid the multiple allegations of bullying and the charity he co-founded. (AP/AAP) (AP/AAP)

“We are all deeply sorry. I hate bullying, can’t stand it,” Bono told the newspaper.

“The poorest people in the poorest places being bullied by their circumstance is the reason we set up ONE.

“So to discover last November that there were serious and multiple allegations of bullying in our office in Johannesburg left me and the ONE board reeling and furious.”

Some former employees have launched legal action against the charity, which aims to tackle poverty and disease- particularly in Africa.

Gayle Smith, ONE’s chief executive since March last year, said a probe found evidence of “unprofessional conduct” as well as “bullying and belittling of staff” between late 2011 and 2015.

“Staff were called names, and some said their manager put them to work on domestic tasks in her home,” she said in a statement.

“The investigation also found the situation was not adequately addressed nor resolved by executive management at the time, and that ONE’s board was not, in my view, properly or fully informed.

“In fact, if they would agree, I would like to meet them and apologise in person.”