More teachers are quitting the profession than at any point in the past 10 years, fuelling fears that a recruitment shortfall is being compounded by retention problems.

About one in 12 full-time teachers are leaving the profession every year, according to the latest figures. Teachers have cited excessive workload, the pressures of inspection and the relentless pace of change as reasons for quitting.

Teaching unions have warned that if the issue is not dealt with it could lead to increased class sizes and an even greater burden on the staff who remain.

Figures from the Department for Education show that in the 12 months to November 2013 - the most recent year for which statistics are available - almost 50,000 qualified teachers in England quit the state sector. The problem is exacerbated by the fact that more than 100,000 potential teachers have never taught despite completing their training.

The findings reinforce the results of a survey by the ATL teaching union, released earlier this week, showing that almost three-quarters of trainee and newly qualified teachers have considered quitting, with workload cited as the biggest factor.

Ruth Evans, headteacher of Cantell School in Southampton, said that although working with teenagers had always been demanding, recent reforms had made teaching more challenging. “It is all the curriculum changes, the exam system, how we’re being measured - and Ofsted, of course,” she added. “There are massive national pressures now. And there can be a feeling at times that those in the profession don’t always have the voice they would like.”

The total number of leavers in 2012-13 was 49,373, the highest for 10 years. It represents an increase of more than 25 per cent over four years, and this is the fourth consecutive year that the figure has risen, from a low of 38,850 in 2009-10.

The number of full-time teachers leaving as a proportion of the total teachers in service - recorded by the DfE as the “wastage rate” - is also the highest for 10 years, at 8.7 per cent.

The rise in the number of overall leavers is despite the fact that the proportion of teachers retiring is the lowest for five years.

Christine Blower, general secretary of the NUT, claimed that teachers were leaving the profession because of deteriorating working conditions. “It is no surprise that teachers are voting with their feet,” she said. “A combination of unacceptable number of hours worked, a punitive accountability system, the introduction of performance-related pay and being expected to work until 68 for a pension has turned teaching into a less than attractive career choice.”

Education secretary Nicky Morgan launched the Workload Challenge last autumn in response to concerns over the length of the working week in schools and the amount of unnecessary tasks teachers were being asked to undertake. The DfE committed to taking action after the survey of teachers received almost 44,000 responses.

Chris Sloggett, who recently left his job as a history and politics teacher after three years in the profession, said too much effort was wasted on tasks that had nothing to do with children’s learning.

The government figures also show that 221,000 qualified teachers under 60 who have previously taught in a school are no longer teaching. Another 106,000 qualified teachers under 60 have never taught in the classroom.

Ofsted’s annual report last month revealed that the number of new entrants to the profession had fallen by 17 per cent over five years, while DfE figures show that 7 per cent of teacher training places for this academic year went unfilled.

A DfE spokesman said the latest figures showed a rise in the number of new teachers, from 50,906 in 2011-12 to 53,329 in 2012-13. “Teaching continues to be a hugely popular career, with more teachers in England’s classrooms than ever before and record levels of top graduates entering the profession,” the spokesman argued.

Three-quarters of new recruits were still in the profession after five years of service, he added. “But we are not complacent. We recognise that as the economy improves, we must continue to attract the best candidates.”

Ms Evans said a supportive culture and the new pay structure, which gave headteachers scope to reward teachers, could help to prevent staff from leaving. “If you know you have good colleagues who have the skills and knowledge to support you, it makes a big difference. It gives you a sense of being part of a team working for children,” she said.

`It’s about serving the people above you’

Chris Sloggett spent three years in the classroom, but left his job as a history and politics teacher at Fortismere School in North London last summer.

“Teachers make a huge difference to children’s lives, and in many ways it is an absolutely fantastic job,” he says. “But I felt like the whole purpose had become about serving the people above you rather than serving the kids.

“Ofsted is a part of that, but it was more about the culture that is promoted by the fear of Ofsted. We’re told we should do something because Ofsted might like it.

“Workload was an issue in my newly qualified teacher year, but by the time I left I had found a way of managing it. Teachers don’t mind working hard if it is going to make a difference to kids’ lives, but if it is just to satisfy someone above you it is another matter.

“I would check my emails at the end of the day doing a very tiring job, and there were all these emails giving me things to do which were not going to help my kids. Each one might be only a little bit of work, but if you have 30 five-minute tasks it all adds up.

“Suddenly you have got this whole other job to do that is nothing to do with the kids you’re teaching.”