Young girls in Liberia are still being sexually exploited by aid workers and peacekeepers despite pledges to stamp out such abuse, Save the Children says.
Girls as young as eight are being forced to have sex in exchange for food by workers for local and international agencies, according to its report.
The agency says such abuse is becoming more common as people displaced by the civil war return to their villages.
The UN in Liberia said it would investigate specific allegations.
The United Nations promised to put safeguards in place after sexual abuse in the refugee camps of West Africa was first revealed four years ago.
This cannot continue - it must be tackled
Save the Children
But a study by Save the Children, which involved speaking to more than 300 people in camps for people displaced by the war, found that abuse was still widespread.
The report said that all of the respondents clearly stated that more than half of the girls in their locations were affected.
Girls from the age of eight to 18 years were being sold for sex, "commonly referred to as 'man business'," the report noted.
One 20-year-old woman told the BBC that she had been forced to have sex with a worker for the World Food Programme (WFP).
"This young man had been doing it to most of my friends. And the children too don't have strong minds. They will have sex with him to get the food," Konah Brown said.
But government officials and teachers are also contributing to the abuse, Save the Children says.
Teachers have demanded sex in lieu of school fees, or even just to give good grades the report found.
"This cannot continue. It must be tackled," said Jasmine Whitbread, Save the Children's UK Chief Executive.
"Men who use positions of power to take advantage of vulnerable children must be reported and fired.
"More must be done to support children and their families to make a living without turning to this kind of desperation."
The WFP's Greg Barrow said the organisation would be taking the latest allegations with "the greatest seriousness" and was already taking steps to investigate them.
"The key here is to find what link in this chain of delivering food, and getting it to the people who need it, is perhaps abusing this position," he told the BBC.
The UN's Humanitarian Co-ordinator in Liberia, Jordan Ryan, also said specific allegations would be investigated.
"Unfortunately not all international NGOs have taken it seriously. But it is a clear priority," he said.
"We have never done enough until there's a zero case load. Has enough been done? Not yet. Are we working on it? You bet we are."