The information commissioner Elizabeth Denham is particularly concerned as the information was obtained by rival gang members.
An ICO report into the breach details how since 2014 Newham YOT has shared a redacted version of the Metropolitan Police's "gangs matrix" database to council departments and other organisations, including children's and education services, the National Probation Service, the charity Fight for Peace and the Department for Work and Pensions.
But in January 2017, a YOT manager forwarded an unredacted version containing a raft of personal details such as names, nicknames and addresses of 203 people, 80 of whom were either no longer gang members or were simply a victim of gang crime.
Four months later, it emerged that a photograph of a page of the unredacted version, showing personal details of 50 people, was being circulated among gangs via Snapchat.
"The commissioner has not been able to establish how the individual gang members obtained photographs of the unredacted Newham gangs matrix," says the ICO's report into the breach.
"Nonetheless, she considers that it more likely than not that the matter was ultimately attributable to the wide distribution of the unredacted database by the YOT, without regulation or control."
After the data breach there was a spate of serious gang violence, with victims including those featured in the list.
This included a teenager, named "Chris", who was shot and killed in September 2017. A serious case review into his death by Newham Local Safeguarding Children Board published last year made reference to the council's data breach.
The ICO report does not draw a direct link between the data breach and the violence but says the incidents "are highly relevant to the nature and extent of the harm that could result if personal data of the type contained in the redacted database was not processed under strict controls".
Further concerns raised are a failure by Newham Council to report the breach to the ICO or to have in place a written policy or guidance concerning its handling of the gangs matrix.
The ICO is also critical of a council investigation into the breach, which did not take place until December 2017. No investigation report was produced and this investigation also failed to establish what had been done with the unredacted database, says the ICO.
The ICO investigation into Newham Council is part of a wider inquiry by the regulator into how the gangs matrix is shared.
"We recognise there is a national concern about violent gang crime and the importance of tackling it, said James Dipple-Johnstone, ICO deputy commissioner.
"We also recognise the challenges of public authorities in doing this. Appropriate sharing of information has its part to play in this challenge but it must be done lawfully and safely.
"Our investigation concluded that it was unnecessary, unfair and excessive for Newham Council to have shared the unredacted database with a large number of people and organisations, when a redacted version was readily available. The risks associated with such a transfer of sensitive information should have been obvious."
In a statement, Newham Council said that mayor Rokhsana Fiaz has apologised personally to the mother of the teenager who was killed following the data breach.
"On behalf of Newham Council I accept the seriousness of the unredacted gangs matrix list being distributed on this single occasion in January 2017 and am sorry that it happened," said Fiaz.
"While there were information sharing protocols in place at the time, clearly they could have been better. The Information Commissioner has recognised that the breach was not deliberate and we welcome that.
"Since becoming mayor in May last year, I have been embedding an enhanced culture of safeguarding across the organisation and this includes the internal control of sensitive safeguarding data in line with ICO requirements and new data protection regimes.
"The council is committed to working with our trusted multi-agency partners to make Newham a child-centred borough where young people can feel safe and protected. The findings of the commissioner further underlines my commitment to that."