Thousands of school exam results may be delayed this summer after a union started industrial action this morning.
Nipsa blamed a long-running dispute with exams body the Council for the Curriculum Examinations and Assessment (CCEA) for its action.
It centres on staff working additional hours and at weekends, and has dragged on for three years, Nipsa said.
"It is regrettable this issue may impact on thousands of students – but this issue is not of Nipsa's making and could easily be resolved," said Nipsa's Alison Millar.
She said in order to ensure the exams process works smoothly for pupils taking GCSEs and A-Levels, some staff are required to work in the evenings and Saturdays to meet with teachers involved in exams.
Union official Kevin Kelly claimed CCEA was "attempting to blackmail members into reducing the Time Off in Lieu (TOIL) arrangements, which have been in place in CCEA for many years".
"Our members only turned to the issue of industrial action and voted to be balloted as there was no alternative left open to them," he said.
Ms Millar said the Labour Relations Agency had intervened.
"Despite this intervention, to date no response has been received (from CCEA)," she said.
"From Friday no Nipsa member will be working TOIL, which means no evening or weekend work will be carried out by our members."
However, the CCEA denied the action would have any effect. It said it had agreed to award TOIL at time-and-a-half on Saturdays, removed the requirement for Nipsa members to have evening meetings, and agreed to consider extending the 12-week period within which TOIL leave might be taken. Chief executive Richard Hanna said the action did not apply to contracted teams of expert examiners.
"The action is being taken by a group of staff who provide support to these teams and only applies to meetings outside of normal working hours," he said.
"Contingency plans are in place to ensure CCEA's examiners continue to receive the support they need to undertake and complete this very important work.
"The matter in question was raised with us in March. Negotiations to resolve the concerns of staff have been ongoing and good progress has been made. We are committed to continue to work hard, in a consultative and inclusive manner, to resolve outstanding issues."