Oxfam faces crisis talks with the Government as it deals with the fall-out following claims of sexual misconduct by aid workers.
International Development Secretary Penny Mordaunt will meet the charity on Monday, after warning the "scandal" had put its relationship with the Government at risk.
Oxfam is facing mounting criticism over its handling of sex allegations, but has denied it tried to cover up the use of prostitutes by workers in Haiti in 2011.
Ms Mordaunt said on Sunday that the charity had lied and failed in its "moral leadership" by failing to fully disclose details of its investigation into the misconduct to relevant authorities.
"I am affording them the opportunity to tell me in person what they did after these events and I'm going to be looking to see if they are displaying the moral leadership that I think they need to now," she told BBC1's The Andrew Marr Show.
"If the moral leadership at the top of the organisation is not there then we cannot have you as a partner."
Charities, including Oxfam, have been told they will have funding withdrawn if they fail to comply with authorities over safeguarding issues.
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.
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.
Ms Russell 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.
Asked if the problem of exploitation could be more widespread in the charity sector, Ms Russell said: "The charity sector is not immune from these sorts of allegations and incidents happening. We have about 1,000 incidents a year reported to us by charities involving safeguarding issues.
"What is really important - and perhaps this hasn't happened in the past so much - is that there is a light of transparency and accountability on it."
Ms Mordaunt said the charity had also "categorically" stated to the DfID that beneficiaries were not involved in the misconduct and no harm was done.
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.
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