Women in Syria have been sexually exploited by men delivering aid on behalf of the UN and international charities, the BBC has learned.
Aid workers said the men would trade food and lifts for sexual favours.
Despite warnings about the abuse three years ago, a new report shows it is continuing in the south of the country.
UN agencies and charities said they had zero tolerance of exploitation and were not aware of any cases of abuse by partner organisations in the region.
Aid workers told the BBC that the exploitation is so widespread that some Syrian women are refusing to go to distribution centres because people would assume they had offered their bodies for the aid they brought home.
One worker claimed that some humanitarian agencies were turning a blind eye to the exploitation because using third parties and local officials was the only way of getting aid into dangerous parts of Syria that international staff could not access.
The United Nations Population Fund (UNFPA) conducted an assessment of gender based violence in the region last year and concluded that humanitarian assistance was being exchanged for sex in various governorates in Syria.
The report, entitled "Voices from Syria 2018", said: "Examples were given of women or girls marrying officials for a short period of time for 'sexual services' in order to receive meals; distributors asking for telephone numbers of women and girls; giving them lifts to their houses 'to take something in return' or obtaining distributions 'in exchange for a visit to her home' or 'in exchange for services, such as spending a night with them'."
It added: "Women and girls 'without male protectors', such as widows and divorcees as well as female IDPs (Internally Displaced Persons), were regarded as particularly vulnerable to sexual exploitation."
Yet this exploitation was first reported three years ago. Danielle Spencer, a humanitarian adviser working for a charity, heard about the allegations from a group of Syrian women in a refugee camp in Jordan in March 2015.
She conducted a focus group with some of these women who told her how men from local councils in areas such as Dara'a and Quneitra had offered them aid for sex.
"They were withholding aid that had been delivered and then using these women for sex," Ms Spencer said.
"Some had experienced it themselves, some were very distraught.
"I remember one woman crying in the room and she was very upset about what she had experienced. Women and girls need to be protected when they are trying to receive food and soap and basic items to live. The last thing you need is a man who you're supposed to trust and supposed to be receiving aid from, then asking you to have sex with him and withholding aid from you."
She continued: "It was so endemic that they couldn't actually go without being stigmatised. It was assumed that if you go to these distributions, that you will have performed some kind of sexual act in return for aid."
A few months later, in June 2015, the International Rescue Committee (IRC) surveyed 190 women and girls in Dara'a and Quneitra. Its report suggested about 40% had said sexual violence took place when they were accessing services, including humanitarian aid.
An IRC spokesman said: "The assessment concluded that sexual violence was a widespread concern, including when seeking access to various types of services across southern Syria. These services included the distribution of humanitarian aid."
The reports - both of which have been seen by the BBC - were presented at a meeting of UN agencies and international charities hosted by the UNPFA in the Jordanian capital, Amman, on 15 July 2015.
As a result of this meeting, some aid agencies tightened up their procedures.
The IRC said: "Within our own operations, we launched new programmes and systems to better protect women and girls in southern Syria. Those programmes continue to be funded by a range of donors, including DfID (the UK's Department for International Development)."
Abuse 'ignored for years'
The charity Care expanded its monitoring team in Syria, set up a complaints mechanism and no longer hands over aid to local councils.
It also asked various UN agencies, including the UN Office for the Co-ordination of Humanitarian Affairs (OCHA) and the UN High Commissioner for Refugees (UNHCR), to investigate further and set up new reporting mechanisms. But Care was refused permission to carry out studies in Jordanian refugee camps.
Ms Spencer claims the aid sector turned a blind eye to ensure that aid still got into southern Syria.
"Sexual exploitation and abuse of women and girls has been ignored, it's been known about and ignored for seven years," she said.
"The UN and the system as it currently stands have chosen for women's bodies to be sacrificed.
"Somewhere there has been a decision made that it is OK for women's bodies to continue to be used, abused, violated in order for aid to be delivered for a larger group of people."
Another source who attended the July 2015 meeting on behalf of one of the UN agencies told the BBC: "There were credible reports of sexual exploitation and abuse going on during the cross-border aid delivery and the UN didn't make any serious moves to address it or end it."
A spokesman for UNFPA said it had heard of possible cases of exploitation and abuse of Syrian women in southern Syria from Care. It said it had not received any allegations of abuse or exploitation from the two NGOs it works with in southern Syria. The spokesman also made clear that UNFPA does not work with local councils as implementing partners.
A spokesman for the children's charity Unicef confirmed it was present at the July 2015 meeting. The charity said it carried out a review of its local partners and contractors in southern Syria and is not aware of any allegations against them at this point. But it accepted that sexual exploitation was a serious risk in Syria and said it was introducing a community-based complaints mechanism and more training for its partners.
A DfID spokesman said it was not aware of any cases like this involving UK aid.
"There are mechanisms already in place to raise issues of abuse and exploitation," the department said. "DfID partners in Syria use third party monitors to verify UK aid distributed in Syria."
The spokesman added that any systematic abuse should be picked up by those monitors and reported to DfID.
An Oxfam spokeswoman said it had not been working with local councils delivering aid in the south of Syria in the run up to 2015 and was not doing so today.
"Our work inside Syria has been largely focused on providing large-scale hardware for supplying water to Syrian communities rather than targeting aid at individuals or specific households," she said.
"We did not receive reports about sexual exploitation around aid delivery in 2015, but have a zero tolerance policy on such abuse."
A spokesman for the UNHCR, Andrej Mahecic, said it was "important to understand that in any aid emergency there is a risk of sexual abuse and sexual exploitation, and to abuse somebody who is in need of assistance is despicable".
He added that while the allegations relating to 2015 were "incomplete, fragmented and unsubstantiated" the UN nevertheless had taken some action when they first surfaced.
He said the UN refugee agency had no access to the area of southern Syria where the abuse was alleged to have taken place, but that the agency did seek to bring local partners to Jordan for training.
He continued: "The mere suggestion that the UN can somehow control the situation in a war zone is rather simplistic and disconnected from the reality of what an aid operation looks like in an open and fierce conflict."