Oxfam has been severely criticised by the Charity Commission for the way it dealt with claims of serious sexual misconduct by its staff in Haiti.
Four employees were fired after allegations emerged that prostitutes were used by staff sent to help victims of the earthquake there in 2010.
The commission said the charity failed to spot signs of “poor behaviour” and did not report child abuse claims.
Oxfam apologised, saying what happened in Haiti was “shameful”.
Claims first emerged in The Times last year that Oxfam employees, including former country director Roland van Hauwermeiren, used young prostitutes while based in Haiti after the earthquake.
The newspaper reported that Oxfam was aware of concerns about the conduct of Mr Van Hauwermeiren and another man when they worked in Chad before they were given senior roles in Haiti.
The Charity Commission said some of the organisation’s failings amounted to mismanagement, prompting the regulator to issue Oxfam GB with an official warning.
“What went wrong in Haiti did not happen in isolation,” Charity Commission chief executive Helen Stephenson said.
“Over a period of years, Oxfam’s internal culture tolerated poor behaviour, and at times lost sight of the values it stands for.”
An internal Oxfam investigation in 2011 led to four people being sacked and three others resigning, including Mr Van Hauwermeiren.
But a public report published after the investigation failed to mention sexual exploitation.
‘Abuse of power’
Oxfam’s chair of trustees, Caroline Thomson, said the charity accepted the findings, describing them as “uncomfortable”.
“What happened in Haiti was shameful and we are deeply sorry,” she said.
“It was a terrible abuse of power and an affront to the values that Oxfam holds dear.”
She added that the decision to allow Mr Hauwermeiren to resign without a fuller investigation into his conduct would not be permitted under current policies and practices.
After the claims emerged Haiti banned Oxfam GB from operating inside its borders and thousands of people stopped making regular donations to the charity.
Oxfam has also not been able to bid for Department for International Development funding pending the outcome of the 18-month Charity Commission investigation.