Schools are using unqualified teachers to cover lessons and prepare pupils for exams, according to a new poll.
It suggests that classroom assistants and cover supervisors are among those being asked to take the place of teachers.
The survey of more than 2,000 teachers in England and Wales, conducted by the NASUWT union, found that more than half (58.7%) said unqualified staff are being used as teachers in their school.
Of these, around three in four (73.8%) said that these unqualified individuals are preparing lessons, 97% said that they teach lessons, and around half (50.8%) said these staff members are preparing pupils for tests and exams.
More than eight in ten (84.6%) of those questioned said that their school regularly uses unqualified staff as teachers.
The poll also asked teachers which roles these unqualified staff members held, and around 17% said that they were individuals on courses to gain qualified teacher status.
Others said that these individuals included cover supervisors, higher learning teaching assistants, other teaching assistants and learning mentors.
NASUWT general secretary Chris Keates said: "Parents and the public should be deeply concerned at the results of this survey.
Now when a parent sends their child to school they have no idea who is teaching them.
"Unqualified staff who are not being given the appropriate training, support and remuneration for their responsibilities are also being exploited."
She added: If any suggestion was made that unqualified doctors were let loose on patients there would be public outrage.
"Why should our children and young people, the future of this country, be treated with any less concern?"
Ministers have allowed academies and free schools to hire teachers without qualified teacher status. They say the move will mean schools can hire experts such as scientists, linguists and engineers who have never worked in schools before.
The survey comes as delegates at the National Union of Teachers (NUT) conference in Liverpool called on their executive to promote demands for a qualified teacher for every child in every class.
A resolution passed by the conference said that there should be a campaign to inform parents and the education community about the continuing importance of qualified teacher status and "alert them to the moves of the Government to allow non-qualified teachers to be employed in academies and free schools."
Proposing the motion, James Looker, an NUT member from Bromley said: "Everyone in this room knows the real agenda for the removal of the requirement for qualified teacher status for academies and free schools. It is clearly designed to de-skill the profession and provide teaching on the cheap, to drive down wages and over time, the quality of education available to everyone who cannot afford to send their child to a public school."
He added: The children we teach deserve better. They deserve a fully qualified teacher in every lesson, every day."
A Department for Education spokeswoman said: "It is simply not true to claim that this is about depressing costs. This is about raising standards.
"Independent schools and free schools can already hire brilliant people who do not have Qualified Teacher Status (QTS).
"We have extended this flexibility to all academies so more schools can hire great linguists, computer scientists, engineers and other specialists who have not worked in state schools before.
"We expect the vast majority of teachers will continue to have QTS. This additional flexibility will help schools improve faster, and give head teachers the freedom to hire the person best suited to their school."