Headteachers will consider industrial action over a bid to turn all schools into academies after a union passed a motion opposing government plans.

A motion against full academisation was amended at the National Association of Headteachers annual conference this afternoon to include a demand that the union’s executive consider industrial action as a “last resort” in the fight against the government’s white paper proposal.

Nicky Morgan, the education secretary, was heckled and laughed-at by angry delegates at the conference in Birmingham earlier today, and although many heads at the event run academies themselves, more than 95 per cent supported the amended motion.

The development makes the NAHT the latest union to consider industrial action. The National Union of Teachers and Association of Teachers and Lecturers have also voted to put it on the table.

The wording of the motion commits the executive to “continue to challenge the perception that structural change is a panacea for improvement” and “consider all options…up to and including as a last resort industrial action to ensure the defence of our comprehensive state education system”.

It also requires the union to call for a public inquiry into the effect full academisation will have on England’s school system.

Michelle Sheehy, headteacher of Millfield Primary School in Walsall, said all leaders knew why the full academisation policy was “flawed”.

“There are no credible arguments for the policy,” she told delegates. “I believe that we need to communicate our strength of feeling as strongly as possible. This is the beginning of privatisation of our school system.

“I think we need to consider, do we really want to be the generation of school leaders who allowed this to happen.”

Mike Millman, the union’s regional secretary for the west midlands, accused Morgan of running the ‘department for being economical with the truth’, adding: “Never before have parents been so let-down, lied-to and patronised by this government.”