Nearly 2,500 University of Montreal lecturers say they'll walk off the job on Wednesday, nearly six months after their contract expired.

They had been holding half-day walkouts until now, but they say the slow pace of contract talks gives them no choice but to hold a general unlimited strike.

"A 20th negotiating meeting has, once again, led to nothing concrete regarding our main demands," the CSN union said in a statement.

At issue are salaries, class sizes and pensions.

About 20 per cent of classes will be affected by the strike.

The strike will mostly affect arts and sciences students, the education department, and evening courses for mature students.

Any exams that were to be given by the striking lecturers will be cancelled.