With the Liberal government's bill to ban strikes and freeze the pay of Ontario's teachers moving towards final passage in the legislature, the Canadian Civil Liberties Association has warned it will challenge the legislation in court.

The CCLA said at a news conference in Toronto on Thursday that Bill 115 is unconstitutional. "A forced labour agreement is no more constitutional than a forced confession," said lawyer Stephen Barrett.

The CCLA said it will seek intervener status in any legal challenge to the bill.

"We are concerned that this bill violates the right to meaningful collective bargaining. Why is it necessary, for instance, remove the right to strike before any job action has occurred or even been contemplated," said CCLA director Sukyana Pillay.

The legislation is expected to pass on Thursday.

Earlier this week about 4,000 teachers and their supporters rallied on the front lawn of Queen's Park calling for the minority Liberals to negotiate with them, rather than legislate.

Three unions representing about 45,000 teachers and school workers agreed to the two-year framework agreement with the province, which the government is now trying to impose on the others.

But the three other unions, representing about 191,000 members, oppose the deal and are fighting the legislation.

The bill, which will likely become law with the Progressive Conservatives' backing, would freeze salaries, ban strikes and lockouts for at least two years.

Ontario Premier Dalton McGuinty has defended the bill saying, "we can't afford [to raise teachers' pay] right now."