THE CANADIAN PRESS -- OTTAWA - A senior Canada Post official says mail could be moving by Tuesday if back-to-work legislation wins swift passage.

Mail sorting would resume Monday in preparation for delivery the next day should the bill receive royal assent Sunday, Canada Post chief operating officer Jacques Cote told the Senate.

During a rare Sunday sitting, senators debated legislation that would order locked-out Canada Post employees to return to their jobs.

After giving the bill second reading, members of the upper chamber heard testimony from federal ministers, Canada Post executives and union members.

The session followed a 58-hour marathon filibuster in the House of Commons led by the opposition New Democrats. The Conservative benches erupted in cheers after MPs passed the bill Saturday night.

The government tabled the back-to-work bill last Monday after Canada Post locked out the union in the midst of rotating strikes.

The NDP tried to stall passage of the bill, calling it unfair to the workers.

Many senators also gave the legislation a rough ride, peppering witnesses Sunday with questions about details of the bill, the longer-term implications for Canada Post and the government's overall approach to labour issues.

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

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

Labour Minister Lisa Raitt told the senators the government acted to preserve Canada's economy.

"This is not our first choice in how we would like to see this labour dispute resolved, but the choice is a necessary one," she said.

"Our citizens cannot afford to be left waiting."

Asked if back-to-work legislation would become a standard Conservative approach to labour disruptions, Raitt said, "If it is a matter of national public interest, the government will intervene."