A "national crisis" in teacher numbers is looming, six unions representing teachers and school leaders in England and Wales have warned.
Government limits to teachers' pay and "real terms cuts" to school budgets risk undermining standards, they warn.
"Teachers need a pay rise," they urge, in a joint statement to the School Teachers' Review Body (STRB), which sets their pay. The government said it was attracting "the best and brightest" to teaching.
Teachers' pay increases have been limited to 1% or less for the past five years, and the government aims to keep to this limit for the next four years.
In October, England's education secretary, Nicky Morgan, wrote to the STRB to remind them of this policy.
'False economy'
But the joint statement warns that "as pay and prospects improve in comparable occupations", further pressure will be placed on teacher recruitment and retention.
This means more children will be taught by teachers who are not specialist in the subjects they teach, it adds.
With budgets "at breaking point", schools will struggle to maintain current spending - "let alone afford pay increases", it continues.
"The government must fully fund the necessary pay increases for teachers and school leaders in both England and Wales."
The six unions are:
- The National Union of Teachers
- The Association of Teachers and Lecturers
- The National Association of Head Teachers
- The Association of School and College Leaders
- UCAC, representing teachers in Wales
- Voice
Brian Lightman, general secretary of the Association of School and College Leaders, described four more years of pay austerity as a "false economy".
Kevin Courtney, deputy general secretary of the National Union of Teachers, said teachers "are already leaving in droves and new graduates looking elsewhere for a career".
Mary Bousted, general secretary of the Association of Teachers and Lecturers, claimed 160,000 more teachers would be needed over three years to cope with a rapid increase in pupil numbers.
But she warned unless the picture improved the government would have little chance of meeting this target.
"Schools will have to start increasing class sizes or shutting courses and cutting the subject options available to pupils".
And Elaine Edwards, UCAC's general secretary urged the government to reconsider its pay policies "for the sake of our children and young people and the education system as a whole".
The STRB is expected to make its recommendations on teachers' pay in April.
A Department for Education spokeswoman said it had worked with the profession to "raise the status of teaching", adding that that a record number of highly-qualified graduates and "experienced career changers" were now teaching.
She added: "But we are determined to go further, and recognise that some schools find it harder to recruit the teachers they need, which is why we are expanding the great Teach First and Schools Direct programmes and we are launching the National Teaching Service, which will mean more great teachers in schools in every corner of the country."
