The school leaders' union unanimously carried a motion opposing forced academisation at their annual conference in Birmingham this afternoon.
The motion, which was backed by more than 95 per cent of delegates, follows Nicky Morgan's defence this morning of the plans to turn all schools into academies by 2022.
The education secretary claimed in her speech today that enforcing academy status will not lead to the closure of good rural schools, as many have feared. School would be encouraged to work in "local clusters" she said.
But headteachers remained unconvinced today as they voted "to consider all options open to NAHT up to and including as a last resort industrial action". It is not clear if this would go as far as a strike, although such action is not unknown: the NAHT took part in the first strike in its 114-year history in November 2011 over pensions.
Jonathan Shields, a delegate from the South East region, said: “It is one of the biggest threats to our current imperfect educational system since the 1944 act. It turns everything on its head. Why should it be one size fits all?”
Michelle Sheehy, headteacher of Millfield Primary School in Walsall, called the universal academisation policy “flawed” and added that there are no “credible arguments for the policy”.
But she added: “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?”
Jacquie Sainsbury, a headteacher from Nottinghamshire, said: “We gave Nicky Morgan an opportunity to listen this morning and she didn’t. We have to put our children first. Our children can’t fight this. We have got to fight it for them. Our staff need us to fight this – they are on their knees.”
Today's vote follows the teaching unions' opposition to the academies plans over the Easter weekend.
The National Union of Teachers (NUT) voted to ballot for strike action against the academisation plans - which could take place in the summer term.
The more moderate ATL also voted to consider "potential industrial action" if the government "continues to impose academisation on England's schools.
Following the vote, Russell Hobby, general secretary of NAHT, said: "We've seen today the serious concerns school leaders have about the government's plans to remove the sensible element of choice about academy conversion, and impose the change on all schools regardless of how well they perform.
"Academy status is appropriate for some but mere conversion doesn't guarantee success. What counts is hard work and a clear plan for improvement - both of which can be achieved without conversion.
"The government has failed to win the argument on academies, so this motion empowers NAHT's negotiators to press them hard on the detail and the rationale for conversion. As things stand the government has no answers to school leaders’ sensible and reasonable questions.
"The Secretary of State has said her door is open and that she will listen. Now she needs to honour that offer."
During Nicky Morgan's speech today, she described academies as the "right step" for the education system.
She said: "The autonomy academy status brings means putting power into the hands of school leaders, because we improve outcomes for young people by ensuring the teachers who teach them, and the heads who lead their schools, are given the freedom to make the right decisions in the interests of those children."
But a number of school leaders shouted "what choice?" at the minister as she spoke about the controversial plans.
A Department for Education spokesperson said: “It is disappointing to hear any union considering industrial action, which holds back children’s education, disrupts parents’ lives and ultimately damages the reputation of the profession.
“The academies programme is at the heart of our reforms which have raised standards for children across the country, including many schools that stagnated under local authority control - 1.4million more children are in good or outstanding schools compared to 2010. We want to work constructively with the sector to ensure every child has the excellent education they deserve."