Legislation that would order an end to postal workers’ rotating strikes has passed the first two phases of the Senate on a rare weekend sitting.

Senators are sitting on Saturday to deal with the bill, which passed the House of Commons during a special session in the early morning hours by a vote of 166 to 43.

Bill C-89 passed the first two readings in the Senate on Saturday afternoon and is now being considered in detail by the committee of the whole.

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After this phase, the Senate will make a recommendation on how to proceed.

If passed, the bill would appoint a mediator-arbitrator to help Canada Post and the union representing its workers come to an agreement. If mediation fails, they would go into binding arbitration.

Members of the Canadian Union of Postal Workers have held rotating walkouts for a month, causing massive backlogs of unsorted mail and packages at postal depots.

Canada Post says it could take weeks — even stretching into 2019 — to clear the backlog that has built up, especially at major sorting centres in Toronto, Montreal and Vancouver.

CUPW’s 50,000 members, in two groups, are demanding better pay for rural and suburban carriers, more job security and minimum guaranteed hours.

Those in favour of the back-to-work legislation say it would help small businesses that rely on parcel delivery around the holidays.

Those against — including the NDP, some of whom walked out of the Commons in protest on Friday evening — say it infringes on postal workers’ right to strike.