Teachers are more than £5,000 a year worse off on average in real terms than in 2010 – according to analysis of official data showing the effect of years of pay restraint on the profession.

With millions of children returning to school this week after the summer holidays, teaching unions said the marked decline in salaries was one of many factors causing an ever more serious recruitment crisis .

The data, released by Labour, and based on school workforce statistics and government inflation figures, shows how teachers’ earnings have been eroded, as annual increases in pay have fallen below the rate of inflation. In 2010, the mean wage paid to teachers in state-funded schools was £34,800. By 2016 this had risen to just £35,100 as the government clamped down on public spending. Assuming the level rises by the 1% maximum permitted under the pay cap, it will hit £35,451 this year. But had the mean salary risen at the rate of inflation (as measured by the consumer price index) every year since 2010 it would now be well above £40,000 a year.

Chris Keates, general secretary of the NASUWT union, said many teachers were enduring hardship. “The latest statistics indicate that more than a quarter are having to rely on credit cards, overdrafts and payday loans to make ends meet every month, and many new and recently qualified teachers are unable to afford to rent or buy a home,” she said.

“To continue to provide high-quality public education for every child, we need a teacher workforce which is competitively remunerated and to restore teaching as the profession of choice for UK graduates..”

According to NASUWT research, 70% of teachers surveyed believed that prospective teachers were put off by the uncompetitive pay, and 82% said salaries were not competitive with other occupations. As a result, 72% said they would not recommend a career in teaching.

The latest figures from TeachVac, a free national job service for schools, show vacancies in London grew by 12% between 2015 and 2017 and by 6% in north-west England, according to a sample of almost 1,000 schools in each area.

Before MPs broke up for the summer, many senior Tories demanded that Theresa May end the 1% cap on public-sector pay rises, which is due to continue until 2020. An announcement on the cap is expected in the budget in November.

John Howson, a former government adviser on teacher recruitment, said shortages were most acute in London because of the higher costs of living and other, better-paid graduate jobs in the capital. “If you carry on paying a maximum 1% rise [to teachers], and everyone else is getting more than 1%, eventually it will have an effect on people’s willingness to become teachers or remain teachers in high-cost areas,” he said.

Pay, along with terms of service and morale, was one of the crucial things affecting teacher recruitment. “The risk to the government is they have knocked away all three of those.”

Kevin Courtney, joint general secretary of the National Education Union, the newly formed largest teachers’ union, said: “Every London authority giving evidence to the national employers’ organisation reported difficulties with recruitment and retention and blamed the high cost of living, particularly housing, driving teachers out.

“This July’s report from the School Teachers’ Review Body confirms that London has the highest numbers of schools reporting teacher vacancies. That’s not surprising when the STRB also reports teachers’ median earnings trail the estimated median earnings of other professionals in London by such a significant margin (more than £7,000 in inner London).”

Angela Rayner, shadow education secretary, said: “It is no surprise that schools are facing a crisis in teacher recruitment and retention when the government has handed teachers a real-terms pay cut year after year.

“These stark figures show that the average teacher is now thousands of pounds worse off than they were in 2010, and the government’s plans to continue with the pay cap will only make matters worse. The consequence is that schools are now struggling to find and keep the staff who run our classrooms. The Tories have missed their recruitment targets five years running and for two years in a row more teachers have left the profession than joined.”

A Department for Education spokesperson said: “There are now more teachers in our schools than ever before – 15,500 more since 2010. Overall the number of new teachers entering our classrooms outnumbers those leaving.

“We take teacher recruitment very seriously which is why we are investing £1.3bn up to 2020 to continue to attract the best and brightest into teaching and have given headteachers freedom over teacher pay, including the ability to pay good teachers more.”

Government sources also stressed that the tax-free personal allowance rose to £11,500 in April so a typical basic rate taxpayer will pay £1,000 less income tax than seven years ago.