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 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".
She added: "I certainly will not let the abhorrent mistakes of a troubling organisation 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".
Ms 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.
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
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.
Oxfam officials are due to meet the Charity Commission on Wednesday.
The commission is also looking into claims that Roland van Hauwermeiren - the charity worker at the centre of the sexual misconduct scandal in Haiti - was employed by Oxfam, two years after he left another aid agency because of concerns about his behaviour.
A former colleague has told the IRIN website that van Hauwermeiren was investigated by British health charity Merlin after it was alleged that he used prostitutes in Liberia in 2004.
Merlin later merged with Save The Children, which says it does not have access to Merlin's records.
"We have immediately contacted Save The Children to establish whether they have any knowledge of and/or hold any records about these reports either at the time of the merger or subsequently and if so to ask them to urgently disclose this information to us," a Charity Commission spokesperson said.
The commission said its own records did not go back to 2004.
Oxfam said that with 10,000 NGOs working in Haiti alone in 2011, "it was unfortunately not possible for Oxfam to ensure that those found guilty of sexual misconduct were not re-employed in the sector".
More than 1,200 Oxfam donors cancelled their direct debits on Saturday, Sunday and Monday. On average there are 600 cancellations per month.
But Oxfam said there had been an increase in one-off donations on Tuesday.
The organisation received 78 single gifts, the highest of any day this year, and 44 new regular gifts (direct debits), the highest since March 2017.
International Development Secretary Penny Mordaunt said she would "not hesitate" to cut government funding to charities that failed to put robust safeguarding measures in place.
At an event in Stockholm to find ways to end violence against children, she said: "No organisation is too big, or our work with them too complex, for me to hesitate to remove funding from them if we cannot trust them to put the beneficiaries of aid first."
Ms Mordaunt, who is also due to speak to the National Crime Agency about the issue, said Oxfam's failure to deal with the actions of some of its staff should be a "wake-up call" to the sector.
"It was not just the processes and procedures of that organisation that were lacking but moral leadership," she added.
Ms Driver is the first of Oxfam's celebrity supporters to publicly address the scandal. But some companies have spoken about their relationship with the charity.
The DofE Award said in light of the allegations, it would "be reviewing our association with Oxfam as a participant volunteering provider". Teenagers can volunteer in Oxfam charity shops through the award.
The programme said it had not received notification of any safeguarding incidents involving DofE participants volunteering in Oxfam stores.
M&S, which has worked with Oxfam since 2008, said in a statement: "M&S continues to monitor the situation very closely as we seek to understand the steps that Oxfam is taking to address them and develop a robust safeguarding plan for the future."
Since the allegations came to light, Oxfam's deputy chief executive, Penny Lawrence, has resigned over the charity's handling of the misconduct.
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.