Rotherham Council covered up the loss of personal data on computers which contained information about the victims of child sexual exploitation (CSE) when 21 laptops were stolen, according to the Casey Report.
Louise Casey's report found that 21 laptops were stolen from the council's Norfolk House building in October 2011.
Today's report said the inspectors had been contacted by a whistle-blower - a former IT employee - who claimed "50% of children's data held by the council at the time" was lost in the theft but the information commissioner was not informed.
Ms Casey said: "The council admitted that some sensitive data was lost, including that relating to victims of CSE. The investigation report shows that the matter was discussed with the police and information relating to CSE was present on the laptops, including the names of adults who may have been offenders.
"This much is agreed between the whistle-blower and the council. But what is in contention is what else was on the laptops."
A lawyer claims more evidence could emerge and the scandal "is not going away." David Greenwood represents 38 of 1,400 victims.
A police officer and two councillors are accused of having sex with victims of the Rotherham child abuse scandal, it has been reported.