The Prime Minister has described the behaviour of staff at scandal-hit Oxfam as "horrific" and demanded that charities reveal their safeguarding arrangements to the Government.
Oxfam's 2011 investigation into the Haiti sex scandal concluded charities should be warned about "problem staff", only for several accused of abuse to take up future posts in the aid sector.
The charity made the recommendations at the end of a report which detailed four dismissals and three resignations over allegations ranging from the use of prostitutes on charity property to sexual exploitation of employees.
Suspicions that underage sex workers had been exploited "cannot be ruled out", according to the document.
Theresa May said: "The behaviour that we've now discovered was horrific.
"It was far below the standards that we expect for the charities and the NGOs that we're working with. And I understand there have been further revelations which show that actually there was physical intimidation of witnesses.
"This is absolutely horrific. This is exactly the problem that we see which means that all too often people don't feel able to come forward to report what has happened to them, the behaviour that they've been on the receiving end of."
On Sunday evening, Oxfam repeatedly refused to clarify whether it had contacted any of the women allegedly preyed upon, either during or since the investigation.
It has instead vowed to meet with the Haitian government more than six years on to apologise for "mistakes" and discuss how to make amends, including to the women affected.
Relief staff had been stationed on the Caribbean island to provide support in the aftermath of an earthquake which killed thousands of people in 2010.
Oxfam officially released the findings of its investigation after a leaked copy was published by The Times, heralding a storm of criticism over how the episode was handled.
It said in a statement: "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 caused."
The report alleges that the director of operations in the country, Roland Van Hauwermeiren, admitted using sex workers in his charity-funded accommodation and was granted a "phased and dignified exit".
Last week he denied using prostitutes in Haiti.
Mr Van Hauwermeiren became a senior figure at Action Against Hunger in Bangladesh, with the charity since claiming Oxfam made no mention of his alleged conduct in 2011.