Delegates at the National Union of Teachers annual conference agreed that teacher shortages and the funding squeeze were the “most serious threats” to the quality of education.

Speakers at the event in Brighton called for the recruitment crisis to be made a priority, rather than “diverting money to politically motivated policies” such as academies and free schools.

But the Department for Education has insisted that there has already been substantial spending on recruitment and that vacancies are not that high.

This afternoon, the NUT backed a motion promising “enhanced support for members up to and including strike action” for teachers facing worse conditions because of a lack of staff.

Mark Taylor, from Birmingham, told delegates he was “part of the problem” as he took early retirement so he “might live to enjoy the rest of it”.

But since returning to “the scene of the crime” as a supply teacher, Mr Taylor said he had gone into schools where a third of staff were on long term supply because they could not recruit.

He added: “It can really be a great career and you all know that. But just at the moment it is hard, it is demanding, and it is a great pathway to an early grave.”

Following the debate, Christine Blower, the leader of the NUT, said: “The causes of the retention problem are clear: workload, workload, workload - for not enough pay.  It is not just the hours worked - though they are too high - but the sheer amount of time spent on an accountability system which functions as though it doesn’t trust teachers.”