A study of the work done by the army of 120,000 teaching assistants has concluded that they are failing to raise classroom standards, and are no substitute for qualified teachers. The research, conducted by a senior government adviser, will further inflame the row over using classroom assistants to cover for absent staff.

The National Union of Teachers is still refusing to sign the national workload agreement, designed to free teachers from administrative tasks, because it believes assistants will be used to replace teachers in some cases.

Writing in Educational Research, an academic journal published by Routledge, professor David Reynolds and Daniel Muijis conclude: “It would seem ill-advised to seek to solve teacher shortages by replacing them with an army of learning assistants unless entry qualifications, training and rewards for the latter are substantially improved.”

Schools, too, need additional training in how best to use the assistants, they suggest. The findings are particularly significant as Professor Reynolds, from Exeter University, is the author of the Government’s Numeracy Strategy. His work on the whole-class teaching methods used in some Pacific Rim countries has led to a version of the same approach in England.

John Bangs, head of education at the National Union of Teachers, said: “It is damning. The issue goes much further than training. Our research on the roles of teacher assistants is that, consistently, surveys of teachers say that support is helpful, but that they are opposed to assistants taking over classes, and covering.”

Reynolds and Mujis, a statistician at Warwick university’s institute of education, base their conclusion on a scheme using teaching assistants to help primary children who struggle with mathematics. The assistants offered extra support to small groups within a class Despite this extra help, their results did not improve.

“Overall, then, this study does not provide much support for the use of classroom assistants as a way of improving the ability of low-achieving students, or as a means of increasing child-adult contact without employing more teachers,” concluded the authors.

Speaking this week, Professor Reynolds said: “Our findings broadly match those from other countries.

“We think that what happens is that in the short term the arrival of classroom assistants makes the classroom complex for the existing teachers.

“The implications of this research are that it would be a mistake to only train the classroom assistants and not also include a large-scale national programme of national training for all the teachers who will be getting classroom assistants.”

Graham Lane, chair of the Local Government Association’s education executive, dismissed the findings.

“They are silly conclusions drawn from inadequate evidence,” he said. “The more adults you have around, the more teachers can use their skills.”