The increased deployment of support staff in schools which was intended to reduce teachers' workload has left them facing extra management duties.
Many find themselves acting as line managers to classroom assistants, a task more demanding than the original administrative duties that support staff were employed to do for them, research shows.
However, the presence of teaching assistants has had a significant impact on classroom control, with teachers spending far less time dealing with poor behaviour.
Researchers said teaching assistants 'can help to clarify and translate information and instructions being given by teachers. This no doubt explains the positive effect on behaviour and participation.'
The study by academics from the Institute of Education in London revealed that support staff have generally had a positive effect on teachers' jobs.
More than 1,500 teachers were questioned about the ways in which they felt that their jobs had changed since the introduction of the government's national workload agreement in 2003. This set out measures to relieve teachers' workload, including extending the role of support staff in the classroom.
Three-quarters of respondents felt this had led to a reduction in the stresses of teaching. They believed that the decrease in administrative and clerical duties had allowed them to concentrate more on pupils and teaching.
But the increased deployment of support staff has left it to teachers to define their own roles and those of their support staff. They act as line managers in the classroom, managing teaching assistants' day-to-day work schedules. They are also responsible for conducting performance reviews and appraisals.
The researchers said: "This had added new tasks to the workload of teachers, which by their nature were more demanding of skills and knowledge than the mainly administrative tasks removed from them in the national agreement."
Many also used teaching assistants for one-to-one work, leading researchers to conclude: "Individualisation of attention was provided by support staff at the expense of teachers."
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said: "The idea that teachers should line-manage staff purely because they're in class together, is neither good for the teacher nor the support staff.
"Schools need to make time for joint planningand proper management to happen."