The BBC has expressed "deep regret" for losing the clothes of "Babes in the Wood" murder victim Karen Hadaway.
Karen's mother, Michelle Hadaway, had given her daughter's clothes to reporter Martin Bashir in 1999 to have them DNA tested by the BBC as part of investigations for the Public Eye programme.
She and Nicola Fellows, both aged nine, had been found sexually assaulted and strangled in a woodland den in Brighton in October 1986.
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Director-general Tim Davie wrote to Julian Knight, chairman of the Department for Digital, Culture, Media and Sport (DCMS) Committee, saying that a BBC review concluded that "regrettably thirty years on, little more can be done to find the missing clothes".
"The distress caused to the families is a matter of very deep regret," he said.
In 1987, Russell Bishop was found not guilty of the girls' murder.
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In 2002 and 2004, Nicola's uncle Ian Heffron contacted the BBC to get the clothes back so they could be used in a retrial.
BBC investigators could not find the items.
Mr Davie said: "We hope that it is some little consolation that the police have confirmed that all forensic evidence needed had been already obtained from the clothing in 1986 and stored separately and therefore the unavailability of the clothing had no material impact on the investigation or the 2018 prosecution which eventually brought Russell Bishop to justice."
The BBC confirmed the clothes were lost after being given to Bashir, who signed a receipt for them, adding that it was "fundamentally wrong that better care was not taken of the clothing".
"This should never have happened," said Mr Davie.
"We are appalled that it did and extend our sincere apologies to the families, both that the clothing was lost in such circumstances and that we have been unable, both now and in 2004, to give them any answers about what happened to the clothing."
The issue resurfaced in the wake of Lord Dyson's report on the 1995 Panorama interview with Princess Diana, which concluded that Mr Bashir deceived her in order to obtain the exclusive.
Mr Davie had initiated a review of the steps taken in 2005 "to ensure that there was no further action that we could take now that may help to locate them".
As part of the review Mr Bashir said he had no recollection of what had happened to the clothes.
A BBC spokesman said: "The BBC is extremely sorry for the distress this has caused Ms Hadaway and we deeply regret we have not been able to give her any answers about what happened.
"The director-general wrote to Ms Hadaway to offer our sincere apologies for the distress caused to her and her family, and extended that apology to the family of Nicola Fellows."