"I am nothing short of horrified by the allegations against Oxfam International. In no uncertain terms do I plan to continue my support of this organisation 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," she said in a statement on Tuesday.
"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."
Oxfam's leaders are accused of trying to cover up the behaviour of some of its senior staff members deployed to Haiti after the devastating 2010 earthquake.
Aid workers, including the Oxfam country director at the time, Roland van Hauwermeiren, were accused of turning a villa rented by the organisation into a makeshift brothel, with prostitutes wearing only Oxfam T-shirts.
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