Members of the NASUWT teaching union have unanimously backed a motion opposing the government’s plan to turn all schools into academies by 2022, a measure the union describes as the “privatisation” of schools.

The motion also says the union will escalate industrial action, including with local, regional or national strikes, if academies deny members an annual cost-of-living pay rise, reduce teachers’ pay or “unreasonably withhold pay progression.”

The union will also use industrial action in cases where academies “fail to protect conditions of service and advance decent working conditions for members” and if they link teachers’ pay to test results, pupils’ progress or schools’ inspection results, it says.

The motion, which was added to the conference agenda at the last minute in the wake of the government’s announcement this month of its academy plans, is the latest in a series of public shows of opposition to the proposals.

It says the union will “oppose the privatisation of schools, including through the Westminster government’s academisation programme.”

It comes after the leaders of the Conservative, Labour and Liberal Democrat parties in local government joined together to urge the government to rethink the proposals.

In a letter to the Observer newspaper today they said: “There is no evidence that academies perform better than council maintained schools.

“Where a school is failing, there is no question that action must be taken - but converting every school, regardless of performance, to an academy will not tackle those issues.”

Brian Cookson, honorary treasurer of the NASUWT and a teacher at a school sixth form in Staffordshire, told the conference: “In England the public education system has been denigrated, attacks made on the pay and conditions of teachers, support for local authorities has been weakened year on year, the curriculum entitlement narrowed for pupils and students and the rights and involvement of parents reduced.

“The academies programme from the outset has been the vehicle through which this agenda was to be moved forward.”

He said it was “unjustifiable and unsubstantiated” for the government to claim that giving schools more “autonomy” and flexibility over pay would lead to higher pay for teachers.

The NASUWT’s motion says the union “condemns the war on public education that has been waged by Government and administrations throughout the UK since 2010.”