SQA staff are due to walk out on Friday and the union Unite warned of the effect this will have on the coursework marking and external verification process.
The union said about 400 staff are expected to take part in the action, which is part of an ongoing row over pay.
Unite said the offer made by the SQA for 2023 and 2024 would see most workers receive pay rises of 5.75% and 3.15% for the two years respectively.
General secretary Sharon Graham said: “The pay offer made by the SQA represents a brutal pay cut. Our members will not accept being so undervalued by senior management.
“Unite will support our members all the way in the fight for better jobs, pay and conditions at the SQA.”
Unite industrial officer Alison Maclean blamed both the exams body and the Scottish Government for the strike action – accusing ministers of failing to provide the money for a better pay deal.
Maclean said: “Unite has attempted to resolve this dispute through negotiation for months. Even this week we put forward counter proposals which could have averted strike action.
“This was rejected outright by SQA’s senior management, who are in turn blaming the Scottish Government over a lack of money to fund a better deal.
“Let’s be clear that the blame for this situation lies both with the SQA and the Scottish Government, who are the ultimate paymasters.
“Both should be under no illusions that months of industrial action will create a major impact on the coursework marking and external verification process.”
SQA staff are due to take part in a second 24-hour strike on Thursday February 29.
In addition, a ban on staff working overtime, working at weekends, and working additional hours which would lead to them accruing time off in lieu has been in place since February 16 – with this action short of a strike due to run until May 10.
A spokesperson for the SQA said: “Industrial action by Unite began last week. This is regrettable, but we have robust contingency plans in place and can reassure learners that there is no impact on their coursework, exams or grades.
“The majority of SQA staff will be at work tomorrow, working hard to deliver for learners.
“Instead of unnecessary scaremongering, Unite should do the responsible thing and negotiate on a serious basis.
“At our request, they attended a meeting this week designed to break the deadlock, but despite a number of extra benefits being added to the existing deal, Unite again rejected an offer outright, without going to their members.
“Unite’s request for a one-year deal isn’t new and would breach public sector pay policy which we are required to adhere to, along with other public bodies.
“The pay offer on the table represents a total average increase of 7.43% in year one and a further total average rise of 5.19% in year two, including pay progression. It is the best offer possible which is affordable and within the limits of public sector pay.”
A Scottish Government spokesperson said: “While this is a matter for SQA as the employer, the Scottish Government remains in close contact with them and has strongly encouraged resolution talks to resume.
“We have received further reassurances from SQA that learners will not experience disruption to their exams, coursework or grades.”