Staff in further education (FE) colleges across Northern Ireland have begun a week of strike action in a dispute over pay and conditions.
Members of the University and College Union (UCU) in FE are then set to walk out every six days until December.
As the UCU represents the majority of more than 1,700 FE lecturers, the walkout is expected to cause widespread disruption to college courses.
Heads of further education colleges said the strike would hurt students.
Colleges were also facing "significant budgetary pressures", they said.
But the UCU said that since notice of industrial action was served, college employers had "not shown any willingness to resolve the dispute".
UCU official Katharine Clarke said lecturers in Northern Ireland have the heaviest classroom timetable in the UK, which prevents them supplementing their pay with other jobs.
"If the weekly teaching hours were reduced, lecturers could boost their wages working part time in industry, which also provides opportunity for skill development that can only enhance the students' learning experience," she said.