The details of 2,743 children and their adoptive parents were leaked in an email attachment accidentally sent to 77 people by a Newcastle City Council employee last month.
Information leaked in email included details of names, addresses and birthdates of the adopted children.
The council blamed the data breach on ‘human error’, adding it had taken steps to contact all of those involved.
The Information Commissioner’s Office (ICO) is investigating and has the power to levy a fine of up to half a million pounds.
In a statement, the council said: “On 15 June 2017, an employee in the council’s adoption team accidentally attached an internal spread sheet to emails inviting adoptive parents to the council’s annual adoption summer party.
A review of data protection across the authority has also been launched. It is understood that the member of staff who sent the email no longer works for the council.
Direct of People, Ewen Weir, said: “I am truly sorry for the distress caused to all those affected. We are conducting a thorough review of our processes to identify what changes we can make to ensure this never happens again.”
A helpline for those who think they may have been affected has been set up and relevant regulators informed.
Following the news and statement from the Council Tony Pepper, CEO and co-founder, Egress Software Technologies said: “One of the most important points is that this was entirely preventable. While we are all human and accidents can happen, when employees – in any industry – are handling information as delicate as this, steps need to be taken to prevent this kind of incident.
“In cases like this, where sensitive information about children is at stake and where the leak of that data could exasperate already delicate situations, there really is no excuse for a breach.
“Any data like this should be automatically encrypted to ensure that, even if it does get out, it can’t be accessed. But technology isn’t the whole answer. We’re handling more data than ever before and that means more focus needs to be placed on better supporting staff.”
Anyone involved with Newcastle’s Adoption Service who has concerns has been asked to call the council’s dedicated helpline on (0191) 211 5562.