Assistants play an "important and effective" role in raising literacy and numeracy standards and are often used to support pupils rather than cut teachers' workload, according to the Office for Standards in Education report on primary schools.
Its findings raise fresh questions about whether schools have enough resources to relieve teachers of administrative tasks.
Under the workload agreement, more than 20 routine tasks, including photocopying and putting up displays were supposed to be transferred to support staff from September.
Most schools operate a system where administrative tasks are shared between teachers and assistants, the report, which is based on inspectors' visits to schools in 2002 and 2003, says.
"Many schools have invested much in training teaching assistants to meet pupils' needs. Not unreasonably they aim to maximise this investment and therefore do not want to reduce the amount of time teaching assistants spend with pupils," the inspectors said.
David Hart, general secretary of the pro-agreement National Association of Head Teachers, said the emphasis on high quality teaching showed the danger of relying solely on assistants to reduce workload.
"Ofsted's findings will reinforce fears of those who believe that putting support staff into teaching situations is a dangerous move that might undermine standards. We need more teachers and more support staff in primary schools."
John Bangs, the National Union of Teachers' head of education, said: "This is a clear example of mixed messages from the Government. They want assistants to cut workload and help in the classroom but there is not enough money in the system to do both."
The NUT is the only major union not to sign the agreement.