Video report by ITV News Political Correspondent Paul Brand
The Deputy Chief Executive of Oxfam has resigned as the charity deals with the fall-out following claims of sexual misconduct by aid workers.
Penny Lawrence stepped down saying she took "full responsibility" for the behaviour of staff in Chad and Haiti "that we failed to adequately act upon".
International Development Secretary Penny Mordaunt is holding meetings with the charity on Monday, after warning the "scandal" had put its relationship with the Government at risk.
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," she added.
"As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility.
"I am desperately sorry for the harm and distress that this has caused to Oxfam's supporters, the wider development sector and most of all the vulnerable people who trusted us.
"It has been such a privilege to work for such an amazing organisation that has done and needs to continue to do such good in the world."
Oxfam's Chief Executive, Mark Goldring 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."
On Saturday, Mr Goldring denied the under-fire charity deliberately lied to the government in 2011 over sex claims about its workers.
"I don't think Oxfam deliberately lied," he told ITV News.
"I fully recognise that by telling half a story it feels that way.
"And with hindsight we should have told the whole story."
It is claimed that Oxfam workers sexually exploited victims of the 2010 Haitian earthquake disaster and hired prostitutes for sex at homes funded by the charity in Chad in 2006.
Then country director Roland van Hauwermeiren admitted during an internal investigation that he paid women for sex in Haiti.
He resigned along with two other senior members of staff and four others were dismissed.
Haiti's ambassador to the UK, Bocchit Edmond said there was a "culture of cover up."
"That crime should never have been committed," he said. "Your mission was not to exploit, to sexually exploit those girls, your mission was not to go to solicit prostitutes, your mission was to go and help those people in need, not to use your power and advantage to exploit them."
Former International Development Secretary, Andrew Mitchell said that he knew nothing at the time of the investigation in 2011 while he was Secretary of State.
He added: "Although Oxfam reported the matter to the Department for International Development, they did not specify the nature of the misdemeanor.
"Had they done so then it would immediately have been escalated to me as Secretary of State and I would have taken the necessary action immediately."
Charity Commission director of investigations Michelle Russell said that the watchdog was not told the full story at the time Oxfam first investigated allegations of misconduct in 2011.
She told BBC Radio 4's Today programme: "We've made very clear that had the details of what has come out over the last few days been told to us, we would have dealt with this very differently.
"We were categorically told there was no abuse of beneficiaries involved in the allegations. Nor were we told that there were issues or possible issues around possible crimes, including those involving minors.
"What we did know - and it was made public at the time - is that it resulted in the sacking of several members of staff and resignations. We were assured that Oxfam had investigated it fully."
Ms Russell said the Commission would be speaking to Oxfam on Monday and had a full day of meetings with the charity scheduled for Wednesday.
Oxfam continues to deny a cover-up in Haiti, where senior staff including were allowed to resign without disciplinary action after the sex claims came to light.
Four members of Oxfam staff were dismissed and three, including the country director, resigned before the end of the 2011 investigation.
An internal report could not categorically rule out if the prostitutes were underage.
A former aid worker for the UN has told ITV News the sexual abuse of children within the aid industry is widespread.
"It is a complete cover-up, industry wide," Professor Andrew MacLeod said.
"The failure to crack down on training, prevention, detection and prosecution of paedophilia has given a green light to predatory paedophiles to go the aid world all over the world."
Now the spotlight is being shone on other charities, with Save the Children, Christian Aid and the British Red Cross all confirming it dealt with cases of sexual abuse or harassment by its staff in the past 12 months.
Ms Russell said about 1,000 incidents a year are reported to the watchdog and that the charity sector is "not immune" from incidents involving safeguarding issues.
She told ITV News: "Any, even if it is one incident is shocking and appalling. But it is not of a surprise in some respects that there are more allegations coming out now because the charity sector is not immune from this. We've seen this in other parts of society, in other sectors.
"We're trying to say to charities, be transparent and accountable about this."
All three charities said none of the incidents involved with paedophilia.
Former 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.
"There has been in my view, not just a cover-up with Oxfam, there is a denial, a culture of denial in the aid sector about the exploitation and sexual abuse that has taken place historically for decades," she told BBC Radio 5 Live's Pienaar's Politics.
Ahead of its meeting, Oxfam announced a package of measures to improve safeguarding, including improved recruitment and vetting and a new whistleblowing helpline.
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.