More than nine out of 10 teachers who also serve as school exam officers believe their classroom performance has suffered as a result, a survey has revealed.
The finding which will put fresh pressure on ministers to ensure administrative tasks are taken off teachers' hands.
The statistic comes from a survey by the Examinations Officers' Association. It covered only 153 teachers, but Uday Patel, the association's secretary, said its findings were likely to be mirrored nationwide.
He said: "During the exams season, the officer will have to be on hand to start exams or to wind them up. They will not be in front of their pupils.
Even when exams are not on, there will be times when they have to be out of lessons to ring the board or to call parents.
"Then there is all the extra paperwork, which has mushroomed in recent years, the forms that have to be filled in or passed to other teachers. It can only make it more difficult to get through all the marking and paperwork that goes with being a teacher."
Since September, exam administration is supposed to have been taken out of teachers' hands under the Government's drive to cut their workloads.
But Mr Patel said that, in reality, more than half of exam officers were still teachers. Some schools found it hard to find or afford the administrative staff to take on the role. There were also concerns that purely administrative officers enjoyed less respect from teacher colleagues than those who combined the job with teaching.
Most schools try to give teachers time off from lessons to administer exams. But for 37 per cent of those surveyed this amounted to only two hours or less.
The workload agreement does appear, however, to have had some effect. More than a third (37 per cent) said their working conditions had improved in the past year, against 18 per cent believing it had worsened.
The survey provided evidence of the burgeoning cost of the exam system.
Schools are spending an average of pound;63,000 on A-levels, GCSEs and vocational courses each year. For colleges, the figure is pound;364,000.