In 2011, Oxfam stated that it had found misconduct by some of its workers in Haiti, where the group was taking part in recovery efforts after a devastating earthquake, and that those people had left the organization. But Oxfam did not specify who was involved or what they had done wrong, and, as it now concedes, it did not inform the police in Haiti.

Last week, The Times of London reported that the misconduct included hiring prostitutes, pressuring Oxfam’s drivers to procure prostitutes and holding sex parties at the group’s houses. The organization fired four people and allowed three others, including Mr. van Hauwermeiren, to resign while under investigation.

Oxfam acknowledged on Friday that there had been sexual misconduct, and that Mr. van Hauwermeiren was among those who left over the matter, but it has not named the others.

It said its investigation in Haiti, where prostitution is illegal and the legal age of consent is 18, was not able to prove or disprove allegations that some of the Haitians hired were under 18. There have been no claims that Oxfam workers demanded sex in return for aid, an allegation that has at times been made against other groups.

Haiti’s ambassador to Britain, Bocchit Edmond, told the BBC that his government would demand that Oxfam name the officials involved in the episode and turn over its investigative files, “so all legal actions will be taken against those who committed those crimes.”

The group has apologized repeatedly, and it said that the behavior of some of its staff members in Haiti had led years ago to tougher policies and safeguards, but it has also insisted that it did not engage in a cover-up.

Oxfam has said that in 2011, it shared its findings with the Charity Commission, a British government agency, which took no action. But the commission’s chief investigator said that it was not told the full extent of the misconduct, and that it would have acted differently if it had been.