Pupils who receive intensive help from teaching assistants make less progress than their classmates, damning research into a key Labour education policy has concluded.
The more attention students receive from support staff, the worse their attainment in the core subjects of English, maths and science, a major five-year study has found.
Despite massive investment in swelling support staff numbers, there is no evidence that they help children achieve better results, the study by London University's Institute of Education has found.
Provisional figures for the beginning of 2009 show there were more than 358,000 full-time equivalent support staff in schools in England and Wales. Almost 182,000 of these were teaching assistants.
Numbers have increased since workforce reforms were introduced in 2003, which aimed to cut teachers' working hours by creating new roles for support staff.
The bill for support staff in England in 2007-08 was more than pound;4.2 billion.
The research found the changes had cut teachers' stress levels, improved discipline and raised the quality of teaching. But it identified fundamental weaknesses in the way support staff have been deployed.
It has become common-place for support staff, who are not required to hold qualifications, to be put in charge of low-attaining children and those with special educational needs, it found. "The more support children are given, the less interaction they have with the teacher," said Peter Blatchford, who led the team. "It's the routine way in which children in most need of support are separated from the curriculum.
"If you're measuring in curriculum subjects and achievement, it's not surprising children will do less well if they're separated from the mainstream curriculum."
Researchers compared different amounts of support from teaching assistants on pupil progress in 2005-06 and 2007-08, examining test results from more than 8,000 pupils in 153 primary and secondary schools.
In English, they found pupils who had the most support from teaching assistants made about a year's less progress than classmates who received the least support.
Pupils are expected to progress by three national curriculum sublevels every two years. But those who had the most support were up to two sublevels behind.
Christian McAnea, head of education at Unison, which represents more than 200,000 support staff, said the Government and schools have been more worried about "getting bodies in schools" than making sure they were effective.
"These findings are not a surprise," she said. "We hope they will be a wake-up call for the Government and schools. Support staff are left with children who need specific help. They often are not able to give them the expert help they require.
The Apprenticeships, Skills, Children and Learning Bill, currently going through parliament, says that it should be an aim for all people in the children's workforce to hold a level 3 qualification - equivalent to two A-levels.
Alma Harris, pro-director of London University's Institute of Education, has carried out separate research into the impact of support staff, which found they had a positive impact on attainment. "The findings are counter- intuitive, surprising and disappointing when you think of the ways schools have actively engaged support staff in teaching and learning," she said.
A spokeswoman for the Department for Children, Schools and Families said: "It's counter-intuitive to say that support staff do not effect pupil progress. They are the backbone of the teaching workforce. There is clear evidence that there is a positive effect on pupil's progress where teaching assistants are trained and effectively trained to deliver specific support programmes, alongside well-planned lessons - as this research acknowledges.
"The most effective deployment of teaching assistants is patchy - that's why the schools White Paper sets out measures to address the issues and make sure support staff are integral to teaching in every school."