Oxfam's deputy head quit on Monday amid a controversy over aid workers' use of prostitutes in countries where they were giving out humanitarian resources.
Penny Lawrence resigned over what she said was the charity's failure to adequately respond to past allegations of sexual misconduct by some of its staff. It comes after days of crisis at the charity triggered by a report in the Times newspaper alleging that some staff had paid for sex with prostitutes in Haiti in 2011, during humanitarian relief efforts after an earthquake.
'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,' Lawrence said in a statement.
'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,' she said.
While neither confirming nor denying the detailed allegations made in the Times article last week, Oxfam had said that it had held an internal investigation over alleged misconduct in Haiti in 2011.
As a result of that investigation, Oxfam said, four staff members had been dismissed and three, including the Haiti country director who had previously held that role in Chad, had resigned.
'The misconduct findings related to offences including bullying, harassment, intimidation and failure to protect staff as well as sexual misconduct,' Oxfam said in a statement on Friday.
Lawrence's resignation came on the same day that senior Oxfam managers were summoned to a meeting with Britain's aid minister Penny Mordaunt, who has threatened to withdraw government funding from the charity if it did not come clean.
In its last financial year Oxfam received £32 million from Mordaunt's ministry, about 8 percent of its overall income. However, private donations are also likely to be hit by the bad publicity over the Chad and Haiti allegations.
Additional reporting from Reuters.