Head teachers and school governors are being urged not to employ "unqualified" support staff to take classes.
Doug McAvoy: Concerned about unqualified staff
The National Union of Teachers is writing to every school in England and Wales asking them to only use qualified teachers to take whole classes.
This is part of the NUT's campaign against an agreement to cut teachers' workload, signed by the government and most other teachers' unions.
The government says using more support staff will help raise standards.
And it rejects the accusation that the workload agreement will mean the replacement of fully-qualified teachers with less-qualified classroom assistants and support staff.
"Workforce reform is not and has never been about replacing teachers with support staff - that is why there is now the highest number of teachers in schools since 1981, while the number of teaching assistants has doubled since 1997," said a spokesperson for the Department for Education and Skills.
But the NUT says the agreement - and its increased use of classroom assistants - could put children's education in jeopardy.
"Allowing unqualified persons to teach whole classes will not raise standards," the letter, signed by general secretary Doug McAvoy, reads.
"In times when budgets are tight, the power provides an incentive to local education authorities and schools to reduce the numbers of qualified teachers and to employ cheaper staff to take on the work of teachers," says the letter, which was sent on Monday.
The union stresses that support staff "have a vital role in supporting teachers" and it calls for better pay, conditions and training for these members of staff.
"Replacing teachers with support staff, however, is the antithesis of providing support to teachers," the letter warns.
The letter also stresses that the union is not against the employment of people qualified to give instruction on specific skills, such as peripatetic music instructors, foreign language assistants and sports coaches.
"Parents do not want staff who are not qualified teachers taking responsibility for their children's education," said Mr McAvoy.
"Standards cannot be improved nor maintained if schools employ unqualified persons to teach. I am confident that head teachers and governors will support our approach," he said.
But the Department for Education and Skills, which has reached agreement with other teachers' and head teachers' unions, has rejected the NUT's claims that the changes to working practices will mean that "anyone" can teach.
"Far from allowing anyone to teach, the new regulations mean that for the first time, skilled teaching assistants, nursery nurses and other support staff will work within an agreed framework of direction and supervision by qualified teachers," said a DFES spokesperson.
"Every class or group must now have a teacher assigned to teach it.
"Teaching assistants and other support in the classroom means that teachers are freed to teach more effectively and ensure that every child is given the individual attention they deserve," said the spokesperson.