The 48-year-old Londoner said she was “in no uncertain terms” withdrawing her support for the charity after more than 20 years working with them.
Taking to Twitter, she wrote: “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 organisation that I have been raising awareness for since I was nine-years-old.
“I have worked with Oxfam almost exclusively on the rights and plight of women in developing countries.
“Women who have to supplement their factory/manual labour with sex work…There are NGOs who do not cover up the egregious abuses of their employees.”
Oxfam came under pressure to hand over its own files of suspected abusers of children and vulnerable women in earthquake-stricken Haiti to the police.
Earlier this week the charity’s UK bosses were summoned to talks with International Development Secretary Penny Mordaunt, who is threatening to cut off £32million government funding over the scandal.
The Charities Commission has faced questions over why it failed to investigate the full extent of what happened at Oxfam.
Director of investigations Michelle Russell accused Oxfam of holding back details of the allegations, including that children and vulnerable people dependant on aid were allegedly exploited for sex.
Meanwhile the sheer scale of abuse allegations across the humanitarian sector continues to emerge, as a Charities Commission official revealed that 1,000 allegations of misconduct across the charity sector had been reported in the past year.
Helen Evans, Oxfam’s former global head of safeguarding, also told Channel 4 of three new allegations made against Oxfam staff overseas in a single day.
She told Channel 4 News: “There was one of a woman being coerced to have sex in a humanitarian response by another aid worker…
“[There was] another case where a woman had been coerced in exchange for aid and another one where it had come to our attention where a member of staff had been struck off for sexual abuse and hadn’t disclosed that, and we were then concerned about what he might be doing, and that was three allegations in one day.”
The charity has issued an “unreserved apology” to the Government, donors, supporters and the people of Haiti following the revelations.
Announcing her resignation as deputy chief executive on Monday, Penny Lawrence said she took full responsibility for what had happened on her watch and was sorry for the “harm and distress” it had caused supporters, as the charity faced a battle to “rebuild the public trust” following crisis talks with the Government over future funding.