Exam markers say they are being forced to work for free, use premium rate telephone helplines and encouraged to risk their health because of a new online system.
The NUT is taking up the case of members who say the decision by the AQA board to move to computerised marking is damaging their pay and working conditions.
Their main concerns centre on the need to mark pre-checked "seed" answers to make sure they are performing to the correct standard before they continue. AQA has told markers it does not pay for the seeds - a policy that one teacher told The TES had led to a drop of about 10 per cent of markers' income.
The marker of a social science AS-level said she had to mark more than 30 out of 314 answers to a question for free. She estimated that for a single AS-level module, every marker was likely to be carrying out eight to nine hours' work - worth about pound;150 - for free.
"We've never been asked to work for nothing before," she said. "But my guess is most examiners don't know how much they are being paid because the system is so complex."
There is also concern that markers could be pressured into working longer days than is healthy because qualification by marking the seed answers for a particular question is valid only for a single day. So there is an incentive to continue because starting again the next day would lead to more unpaid work.
One teacher told The TES that it had taken her 12 hours to mark a batch of answers in a single day.
"I worry about the implications for our eyesight because you have to stare very intensely at the screen for on-screen marking," she said.
The markers are also angry at the costs they could incur from online marking at home. They have been offered pound;25 by AQA towards the cost of broadband internet, are expected to have a computer that matches a set of detailed specifications, and to call a premium rate helpline to resolve any problems with software used in the process.
John Bangs, the NUT's head of education, said: "These are very real concerns that highlight the deregulated nature of what the Qualifications and Curriculum Authority has described as a cottage industry."
AQA said its fees took the marking of seeds into account, that pound;25 would cover a month's broadband, telephone charges could be claimed back, and it did not expect marking to be completed in a single session.