The head of Oxfam GB will be questioned by MPs today over the sex scandal that has engulfed the charity.
Mark Goldring will appear before the Commons International Development Committee, along with the chair of trustees, Caroline Thomson, and Winnie Byanyima, the executive director of Oxfam International.
Meanwhile it has emerged that Mr Goldring is the subject of an internal investigation at the charity following a complaint made last month over how senior management had responded to requests to re-open a 2010 case involving allegations of sexual abuse.
If follows the release by Oxfam of an internal report into alleged abuse by aid workers in Haiti which revealed three of the suspects physically threatened or intimidated a witness.
The 2011 investigation into the Haiti sex scandal concluded that other charities should be warned about "problem staff", adding that several of those accused of abuse went on to take up future posts in the aid sector.
It details four dismissals and three resignations in the wake of the allegations, which included using prostitutes on charity property, sexual exploitation of employees, fraud, negligence and nepotism.
Suspicions that some of the sex workers were under-age "cannot be ruled out", the document said.
Oxfam has also apologised to the Haitian government.
In a statement it said: "Oxfam is grateful to the Haitian Government for allowing us the chance now to offer our humblest apologies and to begin explaining ourselves and start the long road ahead of re-establishing trust and partnership, given our 40-year history with Haiti and its citizens.
Theresa May described the behaviour of some staff at Oxfam as "horrific".
"It was far below the standards that we expect for the charities and the NGOs that we're working with," she added.
Oxfam officially released the findings after a leaked copy of the report was published by The Times newspaper.
In a statement, it said: "We are making this exceptional publication because we want to be as transparent as possible about the decisions we made during this particular investigation and in recognition of the breach of trust that has been caused.
"We hope this also contributes to rebuilding trust with those who support our work."
The 10-page report alleges that Roland Van Hauwermeiren, director of operations in Haiti, admitted using sex workers in his charity-funded accommodation and was granted a "phased and dignified exit".
Last week, he denied ever having used prostitutes on the Caribbean island.
Oxfam staff had been stationed on Haiti to provide support following the devastating earthquake in 2010 which killed thousands of people.
A section of the report entitled "lessons learned action plan" called for tighter safeguarding across the charity industry to stop disgraced aid workers from moving to new jobs.
It read: "Need better mechanisms for informing other regions/affiliates/agencies of behavioural issued with staff when they move and to avoid 'recycling' poor performers/problem staff."
Several men at the centre of the allegations subsequently took up roles in aid organisations, including Oxfam.
Mr Van Hauwermeiren took up a senior role at Action Against Hunger in Bangladesh, which has claimed Oxfam made no mention of his alleged conduct in 2011.
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Another former staff member was employed by Oxfam as a consultant in Ethiopia months after being sacked, a move described as a "serious error" by the charity last week.
Oxfam faces having its funding threatened and an investigation by the Charity Commission following the revelations.