Legislation ordering Canada Post employees back to work received royal assent late Sunday, setting the stage for the resumption of mail delivery on Tuesday.

Earlier Sunday evening, the Senate approved the government's back-to-work legislation after a vigorous debate in a rare Sunday sitting of the Upper Chamber.

After the debate began, Senators heard testimony from federal ministers, union members and company officials.

Canada Post chief operating officer Jacques Cote told Senators earlier Sunday that sorting would begin Monday in preparation for Tuesday delivery should the bill pass quickly.

After the bill received royal assent, the Crown corporation issued a news release confirming that Canadians can expect to start receiving mail on Tuesday.

Post offices will begin to reopen and resume regular business hours on Tuesday, and mailboxes will be unsealed as soon as possible, the statement read.

"With unprocessed mail in the system and accumulated mail received from other countries that has not yet entered our system, it will take some time to stabilize our operations and to return to our normal delivery standards," Canada Post said in its statement. "Any mail in the system at the time of the work disruption has been secured for processing and delivery."

Less than 24 hours after the House of Commons gave third reading to Bill C-6 by a majority vote, Labour Minister Lisa Raitt told CTV's Question Period on Sunday the government's intent was never to side with management or the union in ending the two sides' prolonged contract negotiation.

She said the bill was passed in the interest of Canadians, echoing what Prime Minister Stephen Harper told reporters on Saturday.

The government gave unionized workers and Canada Post "a solution that is best for the economy," she said.

Raitt, who like many other MPs was forced to sleep over on the Hill during the NDP's record-breaking 58-hour filibuster, said the government's objective was never to quash unions.

"The message (from) this is ‘parties do your own deals,'" she said, otherwise, "this is the solution you're going to get.""

Senators fired tough questions at witnesses who attended the Upper Chamber to testify on Sunday, asking about the bill, its long-term implications for both Canada Post and the union and the government's role in settling labour disputes.

"This is contemptuous in its attitude toward a labour union of any kind," said Progressive Conservative Sen. Lowell Murray.

Liberal Sen. Terry Mercer said the legislation was "the beginning of an attack on public service unions."

Impact on CUPW

On Sunday, Canadian Union of Postal Workers president Denis Lemelin called Bill C-6 an attack on the right to free bargaining.

"What the government is doing is really punitive to the worker," he told CTV News Channel.

Lemelin said he would have preferred if the amendments the New Democrats had introduced had passed. They included:

Removing the clause that calls for an arbitrator to choose between final-offer proposals by the two sides.

Removing the salary terms that will undercut union wages originally offered by management.

Legislation 'inevitable'

On Thursday, a day after talks between Canada Post and the union collapsed, NDP Leader Jack Layton and his MPs began delivering 20-minute speeches to delay voting on the bill.

The tactic was designed to delay what many political experts, including retired Queen's University professor Ned Franks, predicted was the inevitable.

Saturday's vote, Franks told CTV News Channel, suggests to Crown corporations that there are no incentives for employers to offer decent wage increases or to engage in collective bargaining "because the government will impose its own rules."

Franks said the bill shows the prime minister "doesn't engage in diplomacy and accommodation … he is a warrior and he wins his battles."

On June 3, CUPW members began 24-hour rotating strikes across the country, which Canada Post said cost the company tens of millions of dollars in lost business.

The Tories tabled back-to-work legislation on Monday after Canada Post locked out unionized workers on June 15.

There was hope the legislation, which provides workers with lower wages than the previous offer from Canada Post, would spur the two sides to reach an agreement on their own.

Now that the dust has settled, Conservative campaign co-chair Guy Giorno said the legislation is more than fair.

He told CTV's Question Period the 1.5 per cent wage hike is reasonable given that many Canadians will likely receive less than that this year.

NDP adviser Brad Lavigne said the bill reveals just how much the government is willing to side with management.

"This government is willing to legislate parties back to work with less wages than what management was willing to offer," he said. "We think that was wrong and we fought it."

Lavigne said he did not feel defeated with the NDP's unsuccessful attempt to push forward its amendments.