Teachers have spent at least 1.5 million days off work owing to stress and mental health issues, new figures have revealed, amid continued concerns over the increasing pressures they are facing in the classroom.

With long-running concerns about workloads and growing class sizes, new data seen by the Observer suggests that the number of days lost to mental health issues in some council-controlled schools in England and Wales has increased by 7% from the previous year. It is also up by almost a fifth compared to three years ago.

The data came in response to freedom of information requests provided by 143 out of 152 local education authorities in England and Wales. In total, over seven million teacher days have been lost to stress and mental health issues in the past five years. They showed a steady increase, highlighting the pressures that the pandemic put on teaching staff.

Some areas appeared to be more heavily affected. Kent saw 91,679 teaching days lost in 2021-22, more than anywhere else in the country. Hampshire saw the number of days taken off for mental health rise to 28,945 in 2021-22, up a third from the year before.

Munira Wilson, Liberal Democrat education spokesperson who uncovered the figures, said she feared there was a growing mental health epidemic among teachers.

“Far too many teachers are facing burnout from unsustainable workloads and relentless pressure,” she said. “Parents will be rightly worried about the terrible knock-on impact this could have on the education and well-being of our children.

“The new education secretary must set out a clear plan to reverse the years of damage to the mental health and well-being of teachers, and to help recruit and retain the staff we need. The Covid inquiry must also look into the impact the government’s mishandling of the pandemic had on the mental health of teachers and other frontline workers.”

This comes with teaching leaders warning that working pressures, combined with a below-inflation pay rise offer, will intensify a crisis in teacher retention. Most have been offered a 5% pay rise next year – higher than the original 3% offer, but well below the 9.1% rate of inflation. Starting salaries will rise by 8.9%. Unions have said they will consult their members on possible industrial action in the autumn as a result of the offer. According to the Institute for Fiscal Studies, the 5% increase offered to most teachers would be “a total real-terms cut of nearly 12% since 2010”. Schools will have to fund the increases out of existing budgets.

Julie McCulloch, director of policy at the Association of School and College Leaders, said stress and poor mental health had become “a really significant problem”. She said: “The biggest problem is workload and this is often cited, alongside pay, as one of the main reasons we have a very high staff turnover rate in education, with 40% of teachers leaving within 10 years of qualifying.

“During the pandemic, schools and staff had to take on a great deal of additional work. All of this will have left many staff feeling burnt out and we are also hearing that some have come out of the pandemic with a view to reappraising their work/life balance and quitting teaching. This is of huge concern because the situation with teacher shortages is already pretty desperate and it seems likely to get even worse.”

Kevin Courtney, joint general secretary of the National Education Union, said: “The government is failing to address the issues of unacceptable workload, supersized classrooms, punitive accountability measures, stress and pay needed to ensure teaching is a profession that is both attractive to graduates and which keeps experienced teachers in post.”

A Whitehall official said the forthcoming Covid inquiry included a pledge to examine the impact on the nation’s mental health. A Department for Education spokesperson said: “We are incredibly grateful for the continued efforts of teachers and school leaders in supporting pupils, especially over the pandemic. We are taking action to support teachers to stay in the profession and thrive. This includes increasing pay and launching the Education Staff Wellbeing Charter, which commits to reducing unnecessary teacher workload, championing flexible working and improving access to wellbeing resources.”