Employees took to social media to publicise the problems

Bono has apologised after it was alleged the charity he founded is at the centre of bullying claims.

The U2 singer founded One in 2004 with an aim to tackle “extreme poverty.”

However the organisation has become embroiled in controversy after employees from its Johannesburg office took to social media caliming they had been bullied.

It was also claimed one female worker was demoted after refusing to "become intimate" with a government official.

Gayle Smith, the charity’s chief executive, said former employees are taking legal action over their complaints.

She said One had filed a serious incident report to the Charity Commission earlier this month.

An internal inquiry found that a former official subjected junior employees to “verbal or email statements such as calling individuals ‘worthless’, ‘stupid’ and an ‘idiot’, at times doing so in front of third parties,” One said.

It also emerged One’s African wing was not paying taxes between 2010 and 2015, but was instead a "non-resident taxpayer."

Bono said: "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.”

Prominent members of the charity's board include former UK prime minister David Cameron and Sheryl Sandberg, Facebook's chief operating officer.