Scandal-hit British charity Oxfam was reeling on Tuesday after fresh claims of sexual assault and rape, this time between aid workers in South Sudan.
Helen Evans, former global head of safeguarding, also warned of assaults on children volunteering in Oxfam's hundreds of high-street charity shops in Britain.
She accused senior managers of failing to act, heaping pressure on chief executive Mark Goldring just hours after his deputy resigned over a scandal involving aid workers' use of prostitutes in Haiti.
Evans told Channel 4 News of a survey conducted during her 2012-2015 tenure which exposed a "culture of sexual abuse" in some Oxfam offices.
The survey of 120 staff across three countries found between 11% and 14% said they witnessed or experienced sexual assault.
7% of staff in South Sudan, four people, witnessed or experienced rape or attempted rape involving colleagues.
She said she asked to take her findings to the senior leadership team at Oxfam, but the meeting was cancelled and Goldring said that discussing the report would not take things any further.
Later during her tenure, Evans said she received three new allegations in a single day in February 2015, including one woman forced to have sex for aid.
"There was one of a woman being coerced to have sex in a humanitarian response by another aid worker, another case where a woman had been coerced to have sex in exchange for aid, and another one where it had come to our attention where a member of staff had been struck off for sexual abuse and hadn't disclosed that," she said.
Evans, now a local councillor in England, said she "struggled" to understand why senior management did not give her more resources to address the problem.
In a separate issue, Channel 4 cited figures showing seven incidents of "inappropriate conduct with children" in Oxfam's shops in 2013/14.
One case involving an adult volunteer assaulting a child - the minimum age for volunteers is 14 - went to court, Evans said.
Oxfam has been battling accusations it covered up a scandal involving its aid workers in Haiti, which has put government funding at risk and threatens its global reputation.
It has denied covering up misconduct allegations against staff members accused of using prostitutes in Haiti following a devastating 2010 earthquake, but admits it could have been more transparent.
Oxfam deputy chief Penny Lawrence resigned on Monday, saying: "As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility."