(CNN) - A whistleblower has accused the scandal-hit Oxfam charity and its UK regulator of failing to act when told that senior staff paid for prostitutes during the relief operation that followed the devastating 2010 Haiti earthquake.
Helen Evans, Oxfam's former global head of safeguarding, wrote on Twitter that Oxfam's top management and the UK's Charity Commission failed to act on accusations of sexual abuse on the Caribbean island and in the African nation of Chad.
She also said in a television interview Monday evening that she passed on evidence of women being coerced into sex in exchange for humanitarian aid, but no action was taken.
In her Twitter statement, Evans, who is a local politician in Oxford, where the charity is based, said a report she compiled in 2014 found 10% of staff reported witnessing or experiencing sexual assault and 7% of staff in one country reporting rape or attempted rape.
An hour before she was due to present the report to top management, she says she was told that while the allegations were serious, the charity's leadership felt there was nothing more it could do and the meeting was canceled.
Moreover, the senior investigator sent by the charity to Haiti following allegations of misconduct "misused drugs, defrauded Oxfam and was later imprisoned, inevitably impacting how well the investigation was handled," she wrote.
She said she took concerns about inadequate checks being carried out on staff working with young children in Oxfam thrift stores to the Charity Commission but it failed to act.
Oxfam says it regrets that it did not act on allegations of sexual abuse and misconduct by staff raised by a former manager, "much quicker and with more resources," the charity said on Tuesday.
In an interview with CNN affiliate Channel Four News on Monday, Evans said she reported serious allegations in a single email she wrote to her line manager in February 2015. "There was one of a woman being coerced to have sex in a humanitarian response by another aid worker," she said.
Evans told of another case "where a woman had been coerced to have sex in exchange for aid."
She has since resigned from Oxfam.
Oxfam's deputy chief executive Penny Lawrence resigned Monday, saying she took full responsibility for the organization's failure to act. Lawrence was head of the program unit when the accused senior staff members were deployed to Haiti after the 2010 earthquake that killed between 200,000 and 300,000 people.
The aid workers -- including the Oxfam country director at the time, Roland van Hauwermeiren -- were accused of turning a villa rented by the organization into a makeshift brothel, with prostitutes wearing only Oxfam T-shirts. Van Hauwermeiren resigned from Oxfam in 2011. He is also accused of paying for sex in Chad where he was Oxfam country director before going to Haiti.
Van Hauwermeiren has not commented publicly since the news brooke on Friday. CNN's attempts to reach him have been unsuccessful.
In the wake of the scandal, UK Development Secretary Penny Mordaunt promised a crackdown and announced the creation of a special unit and new obligations on charities.
"This unit will be wide-ranging and comprehensive in its remit, looking at safeguarding across UK and international charities, suppliers and the UN and multilateral organizations so that together we can make progress. This will look at how to guard against criminal and predatory individuals being re-employed by charities and abusing again, including the option of establishing a global register of development workers," she said.
On Tuesday, the President of Haiti strongly condemned Oxfam over its handling of sex abuse allegations. In a Twitter post, Jovenel Moise called the actions an "extremely serious violation of human dignity."
"There is nothing more outrageous and dishonest than a sexual predator who uses his position as part of the humanitarian response to a natural disaster to exploit needy people in their moment of greatest vulnerability," he said.
The Charity Commission said in a statement it had "engaged with Oxfam on a formal regulatory basis" to address Evans' concerns when she contacted them in 2015. It said it also met with her again in 2017 "and based on this we opened a formal regulatory compliance case with the charity which has been ongoing since that time."
The commission opened a statutory inquiry on Monday into how Oxfam handled the allegations of sexual abuse in Haiti in 2011.