Unison estimates that around 70 per cent of such staff lose thousands of pounds a year because, unlike teachers, their pay does not cover all school holidays.

Christina McAnea, Unison’s head of education, said that assistants were typically paid for a 32-hour week 40 weeks a year, when in reality they work much longer hours. On average they should be paid an extra 25 per cent. “Schools can no longer go on existing on the backs of low-paid support staff,” she said.

The claim is expected to go to local education authority representatives early next month, with a lobby of Parliament planned for June 5.

Ms McAnea said the claim would need central government support and would have to be phased in but, if the will was there, it could be achieved within three years.

Graham Lane, Local Government Association chair of education, said: “It would mean as enormous increase in the wage bill, so that schools would have to employ far fewer teaching assistants.”

The claim comes as consultations begin on a new career structure for school support staff.

This would split them into four categories: administrative staff; curriculum resource and support workers such as technicians; pedagogical assistants and behaviour support assistants.

Each category would have four grades, rising from induction to management level. Staff on grades two to four would be expected to have a national vocational qualification or equivalent.

Level three teaching assistants would be qualified to supervise classes in a teacher’s absence but only level four pedagogical assistants would use their own specialist knowledge to teach whole classes.

The new grades would not be binding on schools, but Unison hopes their introduction will help improve members’ pay.