Classes are cancelled Thursday for 50,000 students at York University after part-time workers went on strike Thursday.

More than 3,400 contract faculty, teaching assistants and graduate researchers who are members of CUPE went on strike at midnight.

York University officials have said they would cancel classes if the union walked out.

The union said members overwhelmingly supported the unanimous decision of the bargaining team and the union executive to reject the university's final offer.

"It's clear that the university administration is pushing us to go out on strike. Over the last three years, our members have had to work harder with less job security and fewer resources," union chairwoman Christina Rousseau said in a release late Wednesday.

The university said it offered the CUPE members a 9.25 per cent increase over three years.

York has said it will attempt to make up any time lost, possibly by stretching the term into December vacation.

A university official said union members would not be locked out and binding arbitration has been offered.

Teaching assistants earn about $17,386 per year, and pay about $3,700 a year in tuition after tuition rebates have been factored in.

The union had been seeking a two-year contract with a seven per cent raise in the first year, and in the second year, a four per cent raise or cost-of-living increase, whichever is more.

The union's membership does more than 50 per cent of the teaching at York University.