The NAHT heads' union has condemned a move by government solicitors to notify dozens of people who gave evidence on the "Trojan Horse" affair that their identities will be disclosed to lawyers acting for five ex-head teachers facing a disciplinary hearing.
Birmingham City Council has also opposed the move, saying it was “extremely alarmed” by the decision. It has called on the government to intervene to halt the disclosure.
The Trojan Horse scandal emerged in late 2013, when Birmingham City Council received a letter referring to “Operation Trojan Horse”: an alleged plot by some Muslim groups to take over schools in the city and run them on strict Islamic principles.
Former Met Police counter-terrorism commander Peter Clarke carried out a government commissioned investigation in 2014 into several schools implicated in the alleged plot, speaking to dozens of staff anonymously. His inquiry found "clear evidence" of a group of like-minded individuals working to support extremist views in classrooms.
The affair sparked teaching disciplinary action by the National College for Teaching and Leadership, with a hearing into five former head teachers - Monzoor Hussain, Lindsey Clark, Razwan Faraz, Arshad Hussain and Hardeep Saini - adjourned since last year.
All face the principal allegation that on or before March 31 2014, they agreed to "the inclusion of an undue amount of religious influence in the education of pupils" at three schools.
Whistle-blowers' identities and unredacted transcripts are set be disclosed to the individuals’ lawyers, although NAHT lawyers managed to temporarily block any details of nine affected members being handed over.
Rob Kelsall, NAHT senior regional officer, said union lawyers were only given six hours to challenge the disclosure.
He said that, at the time of the Clarke investigation, some head teachers and teaching staff had experienced intimidation and death threats, while others lost jobs.
"There's no excuse for the government to now back away from the assurances these brave individuals were given during the Trojan Horse inquiry,” he added.
"The whole situation beggars belief."
Brigid Jones, Birmingham City Council's cabinet member for schools, said: "We're extremely alarmed that the guarantees of anonymity given to the Clarke report witnesses appear to no longer stand.
"Many of these witnesses only agreed to come forward under strong guarantees of confidentiality, and they and their families could be placed at significant personal risk should their identities and full testimonies be revealed.”
Mr Clarke said he was "appalled" that the confidentiality of those who had given accounts to his inquiry team was being put at risk.
While the names of witnesses could be exchanged between legal advisers and the disciplinary panel, they will not be published in the public domain.
TES understands that the Department for Education is still looking at the legal issues and that a final decision has not yet been made on whether the names are exchanged.
A Department for Education spokeswoman said: “It is critical that all NCTL hearings are fair, just and follow the correct procedures.
"Given these proceedings are ongoing it would not be appropriate to comment further at this stage.”