The aid worker at the centre of the Oxfam sex abuse scandal has revealed that his family have disowned him as he claims that there are "lies and exaggerations" in claims of his prostitute use.
In the letter, Allan tells NGOs they must give fresh assurances over reports of "serious misconduct and sexual abuse" in the sector.
MSF is one of the world's largest non-governmental organisations, with 40,000 staff across the globe.
Oxfam sacked its country director in Haiti a year ago following allegations of "mismanagement" and "inappropriate behaviour", the charity said.
Aid giant Oxfam is the subject of an official inquiry over claims that it concealed the findings of a probe into claims that staff used prostitutes while responding to the Haiti natural disaster in 2010.
It added that it had undertaken an investigation and that Charity Commission had confirmed it had taken "appropriate" action.
Oxfam has denied any cover-up and said that the claims of involvement of underage sex workers were investigated internally and had turned out to be "not proven".
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"And unless you report every serious incident or allegation, no matter how damaging to your reputation - we can not be partners".
Mordaunt called the Haiti prostitution allegations "horrifying".
In the wake of the latest revelations, over 1,200 Oxfam contributors canceled their donations on Saturday, Sunday and Monday.
The staff were based in the country's capital Port-au-Prince in 2010 after a huge natural disaster which killed 220,000 people and left a further 1.5m homeless.
Mr Van Hauwermeiren said there had been "a lot of lies and exaggerations" about his time in Haiti.
The investigation led to the dismissal of four employees and three others being allowed to resign. The people from Oxfam, who I have travelled with and met in various countries since my first trip to Ethiopia in1997, inspired me with their passion, courage, integrity and compassion. "They did not inform donors, their regulator or prosecuting authorities", Mordaunt said. "It was not just the processes and procedures of that organisation that were lacking but moral leadership".
Actress Minnie Driver has resigned from her role as an Oxfam celebrity ambassador and corporate backers demanded accountability as the aid organization sought to address allegations that senior staff members working in crisis zones paid for sex among the desperate people the group was meant to serve.