Schools are using unqualified staff to teach pupils and prepare lessons, according to a survey of teachers.
It also suggests that many teachers believe that the use of unqualified staff is worsening because schools cannot, or will not, pay for qualified individuals.
The poll, conducted by the NASUWT union, asked around 7,000 members for their views on schools using staff that do not hold Qualified Teacher Status (QTS).
It found that just over half of those questioned (53%) reported that there were unqualified staff working as teachers in their school.
Around 81% said that the unqualified staff working as teachers were planning and preparing lessons, with 90% saying that these individuals regularly teach lessons and 61% saying that they were preparing pupils for tests and exams.
The results also show that nearly two thirds (65%) of teachers say that the use of unqualified staff is "getting worse because schools can't or won't pay for qualified teachers."
NASUWT general secretary Chris Keates said: "Parents no longer have the certainty of knowing that when they send their children to school they will be taught by a qualified teacher.
"Our children and young people have been robbed of a fundamental entitlement to be taught by qualified teachers."
The union is due to debate a resolution at its annual conference in Birmingham today calling for the reintroduction of a legal requirement for QTS in all schools.
A separate survey, commissioned by the National Union of Teachers (NUT) found that over four fifths (82%) of parents believe publicly-funded schools should only employ qualified teachers, with 80% saying that they would not want their child to attend a school that did not require its teachers to have professional teaching qualifications.
Under government reforms, academies and free schools can hire staff without QTS - a move that ministers said would allow these semi-independent state schools to take on talented individuals who are experts in their field.
The decision proved controversial, with teaching unions arguing that all state-educated children should be taught by a qualified teacher.
The issue was thrust into the limelight last year when Deputy Prime Minister Nick Clegg disowned the key government education policy, exposing a faultline between his party, the Liberal Democrats and their coalition partners, the Conservatives.
A Department for Education spokeswoman said: "It is entirely right that state schools should enjoy the same advantage that private schools have to bring great linguists, computer scientists, engineers and other specialists into the classroom. It is entirely up to headteachers who they employ and we trust their judgment in getting the mix right for the pupils they know best."
She added: "In fact, the latest teacher workforce census show there are 700 fewer non-QTS teachers in schools than there were in 2010, while the percentage of non-QTS teachers in academies is down from 9.4% in 2010 to 5.3%. Overall the quality of the teaching workforce is rising. A record 96% of all teachers now have degrees or above, meaning there are an extra 43,000 teachers with degree level qualifications in classrooms since 2010."
Shadow education secretary Tristram Hunt said: "Many parents will be shocked to learn that David Cameron has changed the rules to allow schools to appoint unqualified teachers on a permanent basis. This is damaging school standards.
"Improving the education our children receive in our schools means continually improving the quality of teaching in the classroom. Labour would end David Cameron's policy and ensure a qualified teacher in every classroom."
Official figures published last week revealed that around 17,000 "unqualified" teachers are working in England's state schools. This figure includes trainee teachers.
:: The NASUWT survey questioned about 7,000 teachers between March 21 and April 3. The NUT poll, conducted by YouGov questioned 1,526 adults between March 25 and April 3.