A decision to reveal whistle-blowers' identities to head teachers facing disciplinary action linked to the "Trojan Horse" scandal is "astonishing" and "beggars belief", union and school chiefs have said.
Birmingham City Council said it was "extremely alarmed" that lawyers acting for the Department for Education (DfE) had written to notify dozens of people who gave evidence about allegations of hardline Islamic influence in classrooms on the promise of anonymity.
The NAHT teaching union, some of whose members received the DfE letter, said the move could dissuade potential whistle-blowers from speaking out in future.
The council has called on Government ministers to intervene to halt disclosure of individuals' identities to lawyers acting for the five ex-head teachers, who are in the midst of a disciplinary hearing.
Lawyers acting for the NAHT union managed to temporarily block any details of nine affected members being handed over.
Former Met Police counter-terrorism commander Peter Clarke carried out a Government-ordered investigation in 2014 into several city schools implicated in the alleged plot, speaking to dozens of staff anonymously.
Mr Clarke said he was "appalled" that the confidentiality of those who had given accounts to his inquiry team was being put at risk.
His original findings, published in the Clarke Report, found "clear evidence" of a group of like-minded individuals working to support extremist views in classrooms.
But he found no evidence of terrorism, radicalisation or violent extremism in schools.
The "Trojan Horse" allegations, which came light in an anonymous letter, are now widely regarded to have been a hoax, but the fall-out sparked several investigations into Birmingham schools' operation and governance, including Park View Educational Trust (PVET), which has since been broken up.
The inquiries led to a series of teaching disciplinary hearings by professional body the National College for Teaching and Leadership (NCTL).
An NCTL hearing into five former PVET head teachers - Monzoor Hussain, Lindsey Clark, Razwan Faraz, Arshad Hussain and Hardeep Saini - has been 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 PVET schools.
The whistle-blowers' identities and unredacted transcripts are set be disclosed to the head teachers' lawyers.
Rob Kelsall, NAHT senior regional officer, said anonymity had been key to some of its members speaking out.
He added that the letters were received on or about December 22, giving until 4pm on Tuesday January 3 - the first working day back - for recipients to respond.
Mr Kelsall said: "Our lawyers had only six hours to challenge the disclosure."
He had also heard of one example where the DfE had sent the confidential letter to an old address.
The union has asked for 28 days to make further representations to block disclosure.
Mr Kelsall said at the time the Clarke investigation was under way that some head teachers and teaching staff had experienced intimidation and death threats, while others lost jobs.
He added: "There's no excuse for the Government to now back away from the assurances these brave individuals were given during the Trojan Horse inquiry.
"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.
"To present individuals who have already gone through extreme stress to give their evidence with this possibility just before Christmas is nothing short of astonishing."
She added: "All three political parties on the city council are united in calling on ministers to intervene, as this has potentially serious consequences not just for the individuals involved in this case, but for whistle-blowers nationwide."