Prime Minister Stephen Harper and his government violated the Charter of Rights and Freedoms when it imposed back-to-work legislation on postal workers in June, the workers' union is arguing in a new legal challenge.

The Canadian Union of Postal Workers filed the constitutional challenge in Ontario Superior Court on Tuesday, and it argues that the freedom to associate guaranteed in the charter was violated by the federal government's bill that ordered 48,000 Canada Post employees back to work. They had staged rotating strikes before being locked out on June 14, bringing mail service to a halt. The following day, Labour Minister Lisa Raitt said the government would table legislation to end the strike and it was introduced on June 20.

The back-to-work legislation stipulated lower wages than had been in the last offer to employees, a four-year term for the contract, and mandated that outstanding issues between the two sides be decided by an arbitrator. CUPW takes issue with all of those conditions in the legislation and has also launched a separate legal battle on the government's selection of an arbitrator.

"This is a very, very important case. It's an important case because we see now that we have a federal government that is very, very cavalier with the fundamental freedoms of workers today," Paul Cavalluzzo, CUPW's lawyer, said at a news conference in Ottawa on Wednesday. He said the Conservatives don't understand the policies underlying collective bargaining or the long history of the right to strike in Canada.

Conservatives acting on 'ideological grounds,' lawyer says

Cavalluzzo said that the Supreme Court of Canada has ruled in other cases that the freedom to associate in the charter protects the right of collective bargaining and that those legal precedents will work in favour of the CUPW's case. He said he expects this to be "tough" litigation that will likely end up in the Supreme Court, but he is confident about the union's case.

"We feel that at the end of the day the courts will direct the government to cease and desist from intervening in the collective bargaining process as it has done this summer and to respect the fundamental freedoms that are guaranteed in the Charter of Rights," the Toronto-based lawyer said.

CUPW argues that back-to-work legislation violates the charter if it interferes with the ability of workers to associate with each other to improve their wages and working conditions. That was the case with Bill C-6, it argues, because the legislation stipulated wages and the term of the collective agreement between Canada Post and the employees instead of the two parties reaching their own negotiated agreement. The law also ordered the employees back to work and therefore prohibited them from striking, which the union argues was a means of exercising the freedom to associate.

Cavalluzzo said that when the Conservatives intervened in the labour dispute claiming it was harmful to the economy, they did so with little evidence.

"We have a government that is acting on ideological grounds, not on empirical evidence as to the damage that may be caused by the strike," he said.

"You don't act on a whim, you don't act on ideology. If you're going to take someone's rights away, if you're going to interfere with those rights you better have strong and clear evidence that there is the kind of damage that they allege," said Cavalluzzo.

Denis Lemelin, CUPW's president, said it is a fundamental right for workers to negotiate their wages and working conditions and that the back-to-work legislation is "unjust."

"It's the democratic right of the workers that is really attacked in this back-to-work legislation," he said.

The legal challenge is important not only for his members, but for all unionized workers, he said, given the current situation with Air Canada flight attendants who are poised for a strike and facing the threat of legislation from Ottawa to prevent one.

"We know what's happening now for the Air Canada workers, they have the threat on their heads of back-to-work legislation," he said.

Lemelin said that when back-to-work legislation is threatened, the employer quits negotiations and sits back and waits for the government to intervene instead of trying to reach a fair deal with employees.