Teaching unions are preparing to mount a string of legal challenges against new academies if they fail to carry out full consultations over teachers' pay and conditions.
Jerry Glazier, chair of the NUT's action committee, said schools converting this month would not have had time to consult under TUPE (transfer of undertakings) regulations, which protect teachers' contracts.
The warning came as the Government announced that 32 schools have become new academies in time for the beginning of term. When the Coalition formed in May, it claimed a "significant number" would be open by September.
A further 110 schools have had "academy orders" signed by ministers, meaning they are likely to convert over the coming months. Seven of the schools will become the first primary academies.
But Mr Glazier raised the prospect of court battles to ensure that proper consultations with staff are carried out before schools switch to academy status.
"It will have been impossible to consult with staff over the school holidays," he told The TES. "We will be watching carefully and local and national action could take place."
Chris Keates, general secretary of the NASUWT, accused the Government of engaging in "collusion and subterfuge" with some heads and governing bodies to keep their academy plans from parents and staff.
"The idea that a handful of governors or an individual headteacher can make such a serious and irreversible decision without having consulted fully with staff, parents and the local community will shock all right-minded people," she said.
"It is likely that some parents will only find out their school has changed its status when the list is published.
"Those promoting academy status are bankrupt of strong, persuasive arguments. Assertions of vast amounts of additional money for academies have proved to be gross exaggerations."
NUT general secretary Christine Blower described the policy as "something of a failure", with fewer than expected numbers of schools opening this month.
Education Secretary Michael Gove was reported this week to be frustrated with teaching unions and officials in his own department for the lower than expected number of academies opening.
But he said: "This Government believes that teachers and head teachers, not politicians and bureaucrats, should control schools and have more power over how they are run. That's why we are spreading academy freedoms. This will give heads more power to tackle disruptive children, to protect and reward teachers better, and to give children the specialist teaching they need."