British-based Mines Advisory Group (Mag) admitted it had ignored allegations about the "habitual use" of prostitutes by aid workers in the Democratic Republic of Congo (DRC).
"In relation to generic allegations of habitual use of prostitutes by Mag staff in DRC it would seem these were not sufficiently followed up at the time as they should have been and we are very sorry about this," it said in a statement late Saturday.
A whistleblower at the charity said they had regularly witnessed members of staff with prostitutes and had told managers in the capital Kinshasa three times between 2011 and 2013.
Diana was the public champion of landmine charities and worked with Mag shortly before her death in a Paris car crash in 1997.
The claims come as part of a wider sex scandal in the charity sector, triggered by allegations that Oxfam staff exploited Haitians after a devastating 2010 earthquake.
The International Committee of the Red Cross said Friday more than 20 staffers have left the organization since 2015 after "paying for sexual services."
Meanwhile, UNICEF's Deputy Director Justin Forsyth also resigned following complaints of inappropriate behavior towards female staff in his previous post as head of British charity Save The Children.