Forty-nine per cent of the 831 school leaders answering the NUT poll said they were considering their departure. And 85 per cent of those said this was because of the quantity of work they were being asked to do.

Like teachers in the primary and secondary sector - who were also surveyed recently - the majority of school leaders (75 per cent) also said their morale had declined over the past two years.

The survey comes after the three government review groups tasked with devising ways to reduce workload released their reports this weekend.

The poll also found that an overwhelming majority of school leaders do not believe the government should force every school in England to become an academy.

It says that 93 per cent of respondents do not think the government’s approach to academisation is appropriate.

The latest figures come after it emerged that leaders of the Conservative, Labour and Liberal Democrat parties in local government have united to demand that the government drops controversial plans to force all schools to become academies by 2022.

In a letter to the Observer, the parties join forces with independent councillors against proposals that they say are contrary to the wishes of many parents and teachers.

The NUT poll was conducted just days after the government’s white paper on education revealed plans to force all schools to become academies. It also included proposals to scrap the current system of awarding teachers qualified teacher status (QTS) after completing a probationary year in a school.

The survey found that 93 per cent of schools leaders think those employed to teach should hold QTS, and 81 per cent do not think replacing QTS with a new accreditation system is the right response to the shortage of teachers.

Meanwhile, 80 per cent dispute that giving schools more freedom over teachers’ pay and conditions will improve educational standards.

The results of the NUT poll - released on the penultimate day of the union's annual conference in Brighton – also found that almost three quarters (73 per cent) of school leaders say that they are experiencing difficulties in recruiting teachers.

And 70 per cent of school leaders think that their school's funding situation next year will negatively affect educational standards.

Christine Blower, general secretary of the NUT, said: “These findings are bleak and reveal bitter distrust from school leaders of the direction of travel for education policy in England. There are logical reasons why half of these school leaders say that they cannot go on and are thinking of leaving.

“Leaders are clear in the survey that the new reforms floated in the White Paper are not in the name of school leaders. The flagship proposal in the new White Paper – to dictate every school must become an academy – has won the support of only 3 per cent of these school leaders. The government has no mandate for such a risky experiment with the English school model.”

She added: “This government has the wrong priorities. It dresses up every change in the cloak of advancing standards. This is fooling no one. Their strategy of cuts, teacher shortages and far reaching, chaotic curriculum and assessment changes, simply isn’t working.

“This is top-down change, imposed from above, with no logical or evidence based justification. It risks a serious spike in school leader resignations, which is simply too high a price to pay."

A spokesman for the Department of Education said the fact that 70 per cent of all open academies had "voluntarily become one" suggested the concerns raised by the poll were "not shared more widely across the country".



He added that ministers already knew that unnecessary workload was one of the "biggest frustrations for teachers" and the Workload Challenge sought to tackle it.

He added: “Rather than simply opposing our reforms, which have already seen 1.4 million more children in good or outstanding schools than in 2010, the NUT would do teachers and pupils a much greater service by engaging constructively with them.”

