The London Borough of Newham has been fined £145,000 by the data watchdog for revealing personal information on more than 200 people who were on a police database.
The Information Commissioner’s Office found a council employee sent an email to 44 recipients in January last year that contained both redacted and unredacted versions of the 'gangs matrix.'
This had previously been sent to the council by the Metropolitan Police as part of its work with partners to tackle gang violence.
The 44 recipients included the council’s youth offending team and external organisations including a voluntary agency.
The information included individuals' dates of birth, home addresses and information on whether they were a prolific firearms offender or knife carrier, as well as their alleged associated gang.
The investigation found that rival gang members had obtained photographs of the information through the social media.
The borough later experienced an outbreak of serious gang violence whose victims included people whose information had been disclosed.
The ICO said it was not possible to say whether the violence was connected to the data breach but highlighted the 'significant harm and distress' that can be caused when sensitive personal information is not kept secure.
The investigation also found that the council did not report the data breach to the ICO and its own internal investigation did not start until some months after the breach became known.
James Dipple-Johnstone, deputy information commissioner, said: 'We recognise there is a national concern about violent gang crime and the importance of tackling it.
'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.'
The council has apologies for the breach, and said it deeply regrets the error.
Mayor Fiaz said: '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. 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.'