Oxfam's deputy chief executive has resigned over the handling of a sex scandal involving aid workers.
The British charity is accused of concealing the findings of an inquiry into claims staff used prostitutes while delivering aid in Haiti in 2011.
Penny Lawrence said she was "ashamed" and takes full responsibility.
The Charity Commission has launched a statutory inquiry into Oxfam - which denies a cover-up - but details of its scope have not yet been released.
The watchdog says it has concerns the charity may not have "fully and frankly" disclosed everything it knew about the claims.
It comes after an earlier meeting with the International Development Secretary in an effort to protect Oxfam's funding from being cut.
Ms Lawrence joined Oxfam GB in 2006 as international programmes director, leading teams across 60 countries, according to the charity's website.
Reality check: How much money goes to Oxfam?
How will the Haiti scandal affect Oxfam?
Oxfam boss says charity is ashamed
Oxfam did not disclose 'full Haiti details'
"Concerns were raised about the behaviour of staff in Chad as well as Haiti that we failed to adequately act upon," she said in a statement.
"It is now clear that these allegations - involving the use of prostitutes and which related to the behaviour of both the country director and members of his team in Chad - were raised before he moved to Haiti."
The allegations emerged in The Times on Friday, which said Oxfam's country director for Haiti, Roland van Hauwermeiren, used prostitutes at a villa rented for him by Oxfam in the aftermath of the 2010 earthquake.
According to the paper, Oxfam knew about concerns over the conduct of Mr van Hauwermeiren and another man when they worked in Chad before they were given senior roles in Haiti.
Oxfam said allegations that underage girls may have been involved were unproven.
Widza Bryant, who worked in HR for Oxfam in Haiti from 2009 for three years, said she shared "ongoing rumours" about locals being exploited with her boss "on many occasions".
She told BBC News: "There were a lot of rumours on the ground about management and leaders exploiting the locals sexually and in other ways to get jobs and to have good standing."