The Teachers Union of Ireland (TUI) has banned overtime in regional technical colleges and the Dublin Institute of Technology from this week, which is the first week of lectures for most college staff.
Full-time lecturers will only be able to work timetabled hours, mostly in the daytime, and will not be able to teach evening classes, which are paid by the hour.
A TUI spokesman said the ban followed a decision by the union's annual conference to take action against the high level of part-time work in RTCs.
The overtime ban will have a particularly severe impact on colleges in Tallaght, Waterford and Cork, which rely heavily on part-time lecturers to teach evening programmes.