The Crown Prosecution Service and Metropolitan police paid out £550,000 to a teenage court witness after he was put at risk, it has been reported.
The boy was promised anonymity to give evidence about a violent gang attack but his details were inadvertently given to gang members, the BBC reported.
The payout in 2008 is thought to have been one of the largest of its kind and came as the witness's family of three had to be moved for their protection.
On Wednesday, a CPS spokesman said: "This payment relates to a case dating from four years ago where information was passed to the defence, which led to concerns about the safety of a young witness and their family, so that they had to be provided with protective measures.
"The CPS recognised that its actions in this case fell below our accepted standard and, together with the Metropolitan police service, reached an agreed settlement of damages and an apology with the parties concerned.
"We regret that on this occasion we did not provide the support which is normally available to witnesses."
A Met spokesman added: "The Metropolitan police service aims to always provide victims and witnesses with the support they require. "When we get it wrong we acknowledge it with those involved and if appropriate provide compensation."
The BBC reported that the family were relocated into the witness protection scheme after being threatened once details of the boy's identity were disclosed.
It is believed the family launched legal action against the CPS and Met for psychiatric damage, lost earnings and disruption to their lives.
The Met paid damages totalling £550,000, with the CPS paying £350,000, and also paid the family's legal costs, the BBC reported. But the family told the broadcaster that for the young witness, "no sum of compensation would have given him back his youth".
"He will never recover from this experience," they said. "He has lost trust in the police and if he were to witness a similar crime tomorrow, he would simply look away."