The union representing Ontario's college faculty has launched a charter challenge of legislation that ended a five-week strike and allowed 500,000 students to return to class.

The president of the Ontario Public Service Employees Union says the back-to-work legislation introduced by the Liberal government, and supported by the Opposition Progressive Conservatives, violated workers' rights.

"The union's rights and freedoms have been denied," Warren (Smokey) Thomas said in a statement. "OPSEU has the right to freely negotiate a collective agreement with the College Employer Council."

In November, when the legislation was passed, the union vowed to challenge it in the courts. It filed the necessary documentation Tuesday.

The union is now demanding that the collective agreement awarded through arbitration be deemed to have expired and that it and College Employer Council, which represents the province's colleges, return to the bargaining table.

Don Sinclair, CEO of the College Employer Council, said the group appreciates the efforts of the legislators who voted to end the labour dispute.

"The government's actions were necessary to end the strike -- all efforts at the bargaining table had been exhausted," Sinclair said in a statement.

Minister of Advanced Education Mitzie Hunter said she couldn't comment on the charter challenge but added that the focus should be on helping students resume their year.

"I'm very respectful of collective bargaining rights that we have in place here," she said. "I can't comment on this specific issue because it will be considered by the courts. Everyone's focus has to be about the students and their learning and the outcomes for them."

In December, an arbitrator gave the province's 12,000 college faculty members a 7.75 per cent raise over four years.

The arbitrator's decision also included new language on academic freedom, which had been the main outstanding issue between faculty and the colleges.

At the time, both sides cheered the wording of that new contract section.

The union said it "will now allow faculty to speak freely about academic issues without fear of reprisal." The colleges, meanwhile, said the academic freedom section enshrines in a contract the policies that already exist at most colleges.

The new contract also included improved job security for partial-load and full-time faculty and a new government-run task force that will make recommendations on faculty complement, precarious work, college funding, student success, and governance issues, OPSEU said.

Hundreds of thousands of students were kept from class during the strike in the fall, and about 27,500 of the roughly 250,000 full-time students decided to withdraw and receive a tuition refund rather than finishing their semester on a condensed timeline