Exam appeals are set to be "severely delayed" when staff from the Scottish Qualification Authority (SQA) go on strike next month over pay.
Unite the union said 86.7 per cent of its members voted for industrial action to take place on September 8, 15 and 16, while 85.9 per cent also said they would take action short of a strike.
An overtime ban and a ban on accrual of time off in lieu will be in effect from September 8 to November 30.
It comes as members are also concerned about the future of their jobs as the SQA is being scrapped as part of education reforms.
The union said the SQA is due to confirm the results of priority appeals to admissions service Ucas on September 6, while the appeals service for standard appeals opened on August 9 and the deadline for centres to submit these is September 2.
Unite warned industrial action will now disrupt the standard appeals stage of the process and estimates that up to 22,000 such appeals will be "severely delayed".
Alison MacLean, Unite industrial officer, said: "Tens of thousands of student appeals will be severely delayed by Unite's industrial action at the SQA. Our members are disillusioned, frustrated and angry.
"Not only have they been offered a brutal real terms pay cut but they have in effect been locked out of the ongoing discussions surrounding education reform.
"Meaningful dialogue and an opportunity for staff to shape the discussions was promised to us in light of the vast experience of our members.
"Hundreds of workers could see their job roles and conditions being significantly altered yet none of us are any the wiser on what this proposed reform will mean on a day to day basis."
Professor Ken Muir previously put forward several recommendations for reforming Scotland's education and qualifications system, including the replacement of the SQA with a new body.
Three new education bodies will be created - a qualifications body, a national agency for Scottish education, and an independent inspection body.
Sharon Graham, Unite general secretary, said: "Unite’s SQA members have emphatically supported strike action due to a derisory pay offer which is worth as low as 1.7 per cent for some workers.
"There are serious concerns over education reform in Scotland, and specifically what this means for the jobs of our members going forward.
"These concerns have in no way, shape or form been addressed. Unite will always defend our members’ jobs, pay and conditions."
Unite claims the SQA and Scottish Government have failed to meaningfully consult with the trade unions on education reform.
The union said it has written to Education Secretary Shirley-Anne Somerville to repeat its concerns about the issue.
A Scottish Government spokeswoman said: "While this is a matter for SQA as the employer, we remain in close contact with them to ensure that resolution talks resume. We hope that any potential disruption can be avoided through further negotiation.
"The Scottish Government remains committed to ensuring that workers' rights are protected throughout the process of education reform and when the SQA is replaced by a new qualifications body.
"We will continue to engage with SQA management, staff and unions to try to address the concerns raised."
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