Headteachers have overwhelmingly voiced their opposition Theresa May’s flagship policy of creating new grammar schools.

A motion at this afternoon’s NAHT conference urged the union’s national executive to “campaign vigorously to reject the proposed expansion of selection at age 11 or older in the absence of any compelling evidence that it promotes social mobility”.

Instead, it pointed to “robust existing evidence” that greater investment in early years education is “the best driver to combat the effects of disadvantage”.

Rob Campbell, who proposed the motion, said more grammar schools would cause “irreparable” damage to the education system.

He added: “Fundamentally, it will not improve social mobility. The world of 2017 is not the world of the 1950s.”

Judy Shaw, who seconded the motion, said high quality early years education makes a difference and can change lives and narrow the disadvantage gap for children at an early age.

She told delegates: “If we are serious about social mobility, 11 is just too late”.

The motion was passed by 97 per cent, with just 2 per cent of delegates voting against it.

A commitment to new grammar schools is expected to be a key plank of the Conservative’s manifesto for the 8 June election.

Yesterday, Russell Hobby, general secretary of the NAHT, said the union would continue to oppose new grammar schools, even if the Conservatives were elected with such a mandate.