The names of young children "at risk" of radicalisation were mistakenly released to the public, the BBC revealed on Monday.
Waltham Forest Council named seven children, pupils at Greenleaf Primary School in Walthamstow.
The children had answered questions as part of an anti-radicalisation survey about how much they agreed with statements including "it is better to be a dead hero than live impassively", "if a student was making fun of my race or religion I would try to make them stop even if it meant hurting them" and "God has a purpose for me".
The questions were asked as part of Brit - the Building Resilience through Integration and Trust, which the council previously insisted is not aimed at children of any particular faith.
Parent Haras Ahmed made the FOI request, who told the BBC: "It's been a disaster from start to finish. Firstly we're told it's a social cohesion policy and then after various questioning they accept it's a de-radicalisation process and then to release the names of the children in such an insensitive way.
"Any parent in any school - whether they are of a Muslim faith or non-Muslim or no faith - would be appalled by their children's data, such sensitive data, being released to a member of the public."
The council told HuffPost UK that the childrens' names were redacted when it released the information but it was "manipulated by a third party".
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A spokesman said: “Someone has then used their own methods to obtain all of the children’s full names, rather than alert the school or the Council that there was a problem with the information released.
“On behalf of the school, the Council has taken legal steps to secure all copies of the information. The school has informed the families affected of the action that the Council is taking on its behalf, and that the Council has launched a full investigation.”
In May, the council had to defend BRIT, its "cohesion project", after it piloted it in schools with large Muslim intakes. One parent called its questionnaire "Orwellian", The Guardian reported at the time.
At the time, councillors Mark Rusling and Liaquat Ali said it was "doing something every council should be doing to help children explore important issues around identity, values and their place in modern society".
They said: “It’s not targeted at children of any one faith – it’s delivered to whole classes of 10 and 11-year-olds. Schools have helped shape the project’s content, more than 250 children have completed it so far, and there are more schools on the waiting list.
“Evaluating the project, as with anything we do, is key in showing us what impact it has. Children’s individual comments were never going to be looked at or passed on, and the evaluation process was always going to be anonymous.
"We know that we need to go back to the drawing board to get the survey and our evaluation right - and we’ll be doing this with parents and schools.”