The talent and commitment of England's teachers is being "wasted on an industrial scale", a union leader has warned.
The job of being a teacher has become "undoable", with thousands choosing to leave the profession but stay working in schools as support staff, according to Dr Mary Bousted, joint general secretary of the National Education Union.
Her comments come amid continuing concerns among the teaching profession about teacher shortages, particularly in subjects such as physics.
Ministers have pledged to cut teacher workload.
Giving a speech at the NEU (ATL section) annual conference in Liverpool, Dr Bousted also warned the Government that the union is putting it on notice that it must trust and support teachers, and that it must take action on issues such as pay, funding and workload.
She told delegates: "No education system can exceed the quality of its teachers. But in England, we waste, on an industrial scale, the talent, commitment and potential of our teachers.
"More than half of teachers in England leave the profession within 10 years of qualifying.
"They leave pupils who need them; colleagues who rely on them; parents who know that their children's life chances are being denied by the acute shortage of teachers in schools.
"Teachers don't want to walk away from the classroom. They want to stay working with their pupils. That is why so many teachers take a huge pay cut and stay in schools working as support staff."
Dr Bousted said: "Can you imagine hospital consultants saying the stress is too much and now I'm going to be an auxiliary nurse, or I'm going to be a nursing assistant or I'm going to go away from the thing I've been trained to do and stay in the healthcare system with a completely different role and take a two thirds cut in my salary.
"If teachers are choosing to do that, walk away but still want to remain in school, but decimate their salary, then something is very wrong.
"But thousands of teachers are doing that. And the reason they are doing that is because the job of being a teacher has become undoable."
She argued that teachers work more unpaid overtime than any other profession, but that pay is not the main reason that teachers are leaving, adding the key issue is workload.
Dr Bousted also told the conference: "I put the Government on notice, now. It must trust its teachers and support them so that teachers can support their pupils and prepare them for life and work in the 21st century."
The NEU is campaigning on issues including pay, school funding, workload and testing, Dr Bousted said.
She also told delegates: "Damian Hinds would do well to listen to our demands. Because, at the moment, we are asking quite nicely.
"But he should be warned, we will only ask for so long. If we don't get answers, and we don't get the movement we need, we will consult our members whose pent-up discontent will not be held in check much longer."