The union representing Canada's postal workers has mounted a legal challenge against the government's use of back-to-work legislation to restore mail delivery last June.

The Canadian Union of Postal Workers announced Wednesday it will challenge the constitutionality of the bill that forced tens of thousands of Canada Post workers back on the job.

The Harper government introduced the bill after nearly two weeks of rolling strikes by postal workers were followed by a lock-out. The turmoil caused mail delivery disruptions that led to a massive back-log of letters and parcels that took days to clear once service resumed.

Denis Lemelin, CUPW's national president, said the union is taking the government to court in order to protect workers' rights to free collective bargaining, as well as the right to strike.

"This back-to-work legislation was unjust," Lemelin told a news conference Wednesday. "It was the democratic rights of workers that were attacked.

"There is a fundamental principle here -- the freedom of association."

The union has hired noted constitutional lawyer Paul Cavalluzzo to handle the case.

Despite the union's concerns over the erosion of workers' rights, the government argued that maintaining postal service was key to preserving the country's fragile recovery from the economic crisis.

The government has made the same argument in recent days when vowing to introduce a back-to-work bill in the event of a strike by Air Canada flight attendants.

Jana Regimbal, a spokesperson for Labour Minister Lisa Raitt, defended the legislation that restored postal service.

"There was evidence that it was causing serious harm to small businesses across the country," Regimbal told The Canadian Press in an email. She did not address Wednesday's announcement.

Cavalluzzo said part of the case will include asking the federal court to decide whether the right to strike is protected under the Charter of Rights and Freedoms.

He told Wednesday's news conference that in his opinion, the government has become "addicted" to back-to-work legislation, citing the ongoing problem with Air Canada's flight attendants in additional to the postal workers' situation.

"It's an important case because we see now we have a federal government that is very cavalier with the fundamental freedoms of workers today," Cavalluzzo said.

He said the case may eventually be heard by the Supreme Court of Canada.