Local authorities and schools know that many assistants are doing teaching work but are frightened to admit it because they do not want to anger the unions and cannot afford to pay the assistants higher salaries.
The claims have been made by Dr Alan Marr of the Open University, who has contributed to a national study of classroom assistants. He says: "When you walk into schools you see photographs of the staff but they don't include the assistants.
"However, when you go into the classrooms you see how important they are to the children. The situation is very different in Scotland where assistants have a very demarcated role."
Dr Marr says that, although the Government has created a career ladder for assistants, it has "signally failed" to address their terms and conditions. "We are tacitly exploiting these women - 97 per cent of the 80,000 assistants are female."
Nursery nurses, who have their own training system, can earn pound;10,000 to pound;11,000, but full-time classroom assistants generally earn between pound;7,000 and pound;8,000. Most assistants are paid much less than that, however, as they are part-timers.
"When I mentioned the low salaries to one education authority officer he said that if you compared classroom assistants' wages with what you could get in a sweatshop or a restaurant they were well-off. I found his rationale extraordinary."
Dr Marr says tht some authorities are making a determined effort to pay assistants a decent salary and provide proper training but others pay only just above the minimum wage of pound;3.70 an hour. He says assistants at schools he has visited have been made redundant or had their hours slashed. One authority was paying for places in private nurseries if parents wanted them, even though it meant cutbacks for some very good state nurseries.
The Government has said it will hire 20,000 extra assistants at a cost of pound;350 million but Dr Marr is unsure whether this recruitment target will be reached.
"There are questions about the adequacy of the funding and whether it is getting directly to schools. It seems that the new money is being used to retain existing staff."
Dr Marr says the Government should also be concerned about inadequate training - one LEA surveyed by the OU will take 10 years to train all its assistants at its current rate.
The lack of police checks also worries him. "I know of only one school that said it runs checks on the suitability of assistants."
The Government envisages an expanded role for classroom assistants. In a consultation paper - Teaching assistants: good practice guide - it suggests assistants should be able to take control of a class, provided plans are made in advance with the teacher. The document says this would free teachers to focus on pupils who need special help.
UNISON and the National Union of Teachers have both expressed serious reservations about the proposals.
"Classroom assistants in primary schools: from policy to practice", by Alan Marr, Open University.