Minnie Driver has quit as an Oxfam ambassador following allegations senior staff working in crisis zones paid for sex with vulnerable locals.
The actress resigned after working with the charity for 20 years, saying she was "horrified" by the scandal.
"I am nothing short of horrified by the allegations against Oxfam International," she told the Telegraph.
"In no uncertain terms do I plan to continue my support of this organization or its leaders.
"And though it is unfortunate that after 20 years I am no longer able to advocate and defend through this specific framework, social and economic injustice is more globally prevalent than ever.
"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 48-year-old later tweeted: "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"
The Charity Commission announced on Tuesday it has opened a statutory inquiry into Oxfam and International Development Secretary Penny Mordaunt is due to give a speech on Wednesday threatening to cut government funding to charities who fail to put robust safeguarding measures in place.
"The sexual exploitation of vulnerable people, vulnerable children, is never acceptable. But when it is perpetrated by people in positions of power, people we entrust to help and protect, it rightly sickens and disgusts. And compels us to take action," she is expected to say.
"Unless you safeguard everyone your organisation comes into contact with, including beneficiaries, staff and volunteers - we will not fund you.
"Unless you create a culture that prioritises the safety of vulnerable people and ensures victims and whistleblowers can come forward without fear - we will not work with you."
Oxfam has issued an "unreserved apology" to the Government, donors, supporters and the people of Haiti over its handling of incidents including the alleged use of prostitutes by workers, in the earthquake-hit country in 2011.
Four members of Oxfam staff were dismissed and three, including the country director Roland van Hauwermeiren, resigned before the end of the 2011 investigation.
Haitian president Jovenel Moise said on Wednesday: "What happened is an extremely serious violation of human dignity.