Strikes at the Scottish Qualifications Authority (SQA) will go ahead on Thursday, after a new pay offer failed to halt the industrial action.
Unite says up to 55,000 exam appeals could be affected by the strike, dismissing the revised pay deal as a "cynical publicity stunt".
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The strike at the exams body will begin on September 8 and continue next week on September 15 and 16. Union officials also announced six further dates for strike action in late September and early October, as well as an overtime ban.
Earlier on Wednesday, SQA chief executive Fiona Robertson appeared at a Holyrood committee and told MSPs a new offer had been tabled. She said: "I appreciate there's been concern about the impact of potential industrial action at SQA on the appeals process.
"I share that concern. I can confirm that following agreement by the Scottish Government a revised pay offer was put to SQA unions on Monday night and I hope they accept."
Unite said the revised pay offer still represented a real terms pay cut for the lowest-paid staff. The union said for nearly half the workforce the consolidated pay offer was as low as 3.3 per cent.
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General Secretary Sharon Graham said: "Unite's SQA members are set to take different forms of industrial action due to a pay offer which still represents a substantial real terms pay cut for the lowest paid.
"It is completely unacceptable and we will stand up for our members' jobs, pay and conditions at the SQA because they deserve better."
Alison MacLean, Unite industrial officer, said: "Industrial action will go ahead at the SQA and it will continue until a fair pay offer is on the table.
"This will severely disrupt up to 55,000 student appeals. Talks through Acas have barely moved us forward one inch.
"The reason for this is because the SQA have in effect been prevented from tabling an improved offer by the Scottish Government due to the various public sector pay disputes.
"The last-minute offer is nothing but a cynical publicity stunt designed to blame the workers who have no option but to take industrial action due to the procrastination displayed by both the SQA and Scottish Government."
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