The deputy chief executive of UK-based charity Oxfam has resigned over a 2011 prostitution scandal in Haiti, saying she took "full responsibility" for the behaviour of staff there and in Chad, "that we failed to adequately act upon".
In a statement, Penny Lawrence said: "I am deeply sad to announce that I have resigned as deputy chief executive of Oxfam GB.
"Over the last few days we have become aware that concerns were raised about the behaviour of staff in Chad as well as Haiti that we failed to adequately act upon.
"It is now clear that these allegations - involving the use of prostitutes and which related to behaviour of both the Country Director and members of his team in Chad - were raised before he moved to Haiti.
"As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility".
Ms Lawrence said she is "desperately sorry for the harm and distress" caused to the charity's supporters and the "vulnerable people who trusted us".
Mark Goldring, Chief Executive of Oxfam GB, said: "I deeply respect Penny's decision to accept personal responsibility.
"Like us, she is appalled at what happened and is determined to do what is best for Oxfam and the people we exist to help."
"I would like to place on record my sincere thanks for the years of dedicated service that Penny has given to Oxfam and the fight against poverty around the world."
Meanwhile, the EU has ordered Oxfam to explain itself over the scandal in Haiti, warning charities that the union would cut their funding if they breached ethical standards.
Oxfam has been left reeling by reports in the Times newspaper that senior staff hired young sex workers in Haiti after the 2010 earthquake that devastated the island.
The European Union said it took a "zero tolerance" approach to misconduct by NGO partners.
"We expect Oxfam to fully clarify the allegations with maximum transparency as a matter of urgency, and we're ready to review and, if needed, cease funding to any partner who is not living up to the required high ethical standards," spokeswoman Maja Kocijancic told reporters.
The EU provided Oxfam with €1.7 million in funding in 2011, she said.
Oxfam, which employs around 5,000 staff and has 23,000 more volunteers, has announced new measures to tackle sexual abuse cases, though Chief Executive Mark Goldring has insisted there was no cover-up in 2011.
The charity is accused of failing to warn other aid agencies about the staff involved, which allowed them to get jobs among vulnerable people in other disaster areas.
Former British International Secretary Priti Patel has said there was a "culture of denial" about exploitation in the aid sector.
She said she was not aware of allegations within Oxfam, but had raised the issue of abuse involving aid workers in disaster zones with the Department for International Development (DfID) while heading the department.
Four members of Oxfam staff were dismissed and three, including the country director, resigned before the end of the 2011 investigation.
The charity said allegations that under-age girls may have been involved were not proven.
Ahead of its meeting, Oxfam announced a package of measures to improve safeguarding, including improved recruitment and vetting, a new whistleblowing helpline and a recommitment to report concerns to authorities.
Caroline Thomson, Oxfam's chairwoman of trustees in the UK, said: "It is not sufficient to be appalled by the behaviour of our former staff - we must and will learn from it and use it as a spur to improvement."
She added that concerns raised about the recruitment and vetting of staff involved in the scandal were being examined.
Meanwhile, Oxfam Ireland has said described the behaviour of some staff employed by Oxfam GB in Haiti in 2011 as "totally unacceptable".
In a statement, the Irish arm of the organisation said it wished to "completely condemn any form of abuse against the people we work to protect and support".
Oxfam Ireland said it has a zero-tolerance approach to sexual misconduct and has several safeguarding policies in operation to prevent harassment and abuse, including a prevention of sexual exploitation and abuse policy.
It said staff are encouraged to raise any concerns they may have, without reprisal.
Additional Reporting PA