Actress Minnie Driver has stood down from her role as a celebrity ambassador for Oxfam.
This follows claims that staff for the charity in Haiti and other countries paid vulnerable people for sex.
In a statement Driver said that she was "nothing short of horrified" by the allegations.
Oxfam said it was "grateful" for Driver's commitment and that it was "more committed now than ever to learn from our mistakes".
The British charity is accused of concealing the findings of an inquiry into claims staff used prostitutes while delivering aid in Haiti in 2011.
The actress, best known for the films Good Will Hunting, Grosse Point Blank and Hope Springs, said that although she could not continue her 20 years of involvement with Oxfam, she would work against "social and economic injustice".
She added: "I certainly will not let the abhorrent mistakes of a troubling organization stop me or anyone else from working with good people in this space to support a population of human beings around the world that needs our help."
The actress later said on Twitter that she was "devastated by the response" of Oxfam which she had been "raising awareness for since I was nine years old".
All I can tell you about this awful revelation about Oxfam is that I am devastated.Devastated for the women who were used by people sent there to help them, devastated by the response of an organization that I have been raising awareness for since I was 9 years old #oxfamscandal
— Minnie Driver (@driverminnie) February 13, 2018
End of Twitter post by @driverminnie
Driver had been to countries including Cambodia and Thailand in her role as a celebrity ambassador, and also performed at a fundraising concert for Oxfam.
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.
International Development Secretary Penny Mordaunt will give a speech on Wednesday where she will threaten to cut government funding to charities that fail to put robust safeguarding measures in place.
She is attending an event in Stockholm to find ways to end violence against children but is expected to use the opportunity to speak about the Oxfam scandal and say: "Unless you safeguard everyone your organisation comes into contact with, including beneficiaries, staff and volunteers – we will not fund you.
"And unless you report every serious incident or allegation, no matter how damaging to your reputation – we cannot be partners."
Driver is the first of Oxfam's celebrity supporters to publicly address the scandal.
Since the allegations came to light, Oxfam's deputy chief executive, Penny Lawrence, has resigned over the charity's handling of complaints.
In a separate blow for the charity, Oxfam's international chairman, Juan Alberto Fuentes, was arrested in Guatemala as part of a corruption investigation relating to his time as the country's finance minister.