Actress Minnie Driver has stood down from her role as a celebrity ambassador for Oxfamб BBC reports.
This follows claims that staff for the charity in Haiti and other countries paid vulnerable people for sex.
In a statement Ms Driver said she was "nothing short of horrified" by the allegations.
Oxfam said it was "grateful" for Ms 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.
Organisations including Marks and Spencer and the Duke of Edinburgh's (DofE) Award have also said they are considering their association with the charity.
Ms Driver, best known for the films Good Will Hunting, Grosse Point Blank and Hope Springs, said although she could not continue her 20 years of involvement with Oxfam, she would work against "social and economic injustice".