She wrote on Twitter last night: "All I can tell you about this bad revelation about Oxfam is that I am devastated".
It has emerged that a number of people working for the charity paid vulnerable people for sex while working in crisis-hit overseas countries.
Britain's Charity Commission launched a statutory inquiry on Monday, saying it had concerns that Oxfam "may not have fully and frankly disclosed material details about the allegations at the time in 2011, its handling of the incidents since, and the impact that these have both had on public trust and confidence".
Van Hauwermeiren resigned from his role with medical relief charity Merlin in Liberia in 2004 after an investigation into sex parties with local young women, according to humanitarian news website Irin.
Merlin later merged with Save The Children, which says it does not have access to Merlin's records.
The cancellations are double the average cancellation rate of 600 per month, BBC reported. "The case for the type of work done by Oxfam is too strong to allow it to be undermined by bad behaviour", he said. Teenagers can volunteer in Oxfam charity shops through the award.
Penny Mordaunt, the global development secretary, said that she would meet the National Crime Agency (NCA) tomorrow after talks with charity bosses and regulators.
HAITI'S president has condemned the "serious violation of human dignity" by Oxfam following the sex allegations against aid workers in the country.
"As an organisation, we are ashamed by the actions of some of our staff and are more committed now than ever to learn from our mistakes and ensure we uphold the highest levels of safeguarding standards in our work around the world".
An Oxfam store in London.
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".
In another blow to the Oxfam brand, prosecutors in Guatemala said that they had detained Juan Alberto Fuentes Knight, the chairman of Oxfam International.