The union representing about 7,400 workers in the British Columbia port dispute has been warned by the Canada Industrial Relations Board that changing its mind about a new deal during ratification would be an "unfair labour practice."

The board's order issued Sunday also says the union must hold a ratification vote on the deal no later than Friday.

The International Longshore and Warehouse Union Canada and the BC Maritime Employers Association announced a late-night breakthrough Sunday, saying in a joint statement they had reached a new negotiated agreement and would be recommending it to their members.

The joint statement says the new tentative deal was reached with the assistance of the industrial relations board.

The long-running dispute saw workers walk off the job at more than 30 port terminals and other sites for 13 days at the beginning of July, freezing the movement of billions of dollars worth of cargo in and out of some of Canada's busiest ports.

Sunday's warning to the union's leadership comes after a previous proposed contract was supported by union negotiators, then rejected by leaders before they changed course and recommended it to members, who sank it in a full vote last week.

After that deal was voted down by union members, Labour Minister Seamus O'Regan announced Saturday he was directing the industrial relations board to determine if a negotiated end to the dispute was still possible, and if not, to impose an agreement or final binding arbitration.

Pressure had been mounting for federal intervention if a deal failed to eventuate.

Parties including Alberta Premier Danielle Smith, the Business Council of Canada and the Canadian Federation of Independent Business have all urged the federal government to legislate an end to the dispute if it continued.

The industrial relations board's order, posted online, also says the union must not engage in strike activity, and the employers must not conduct a lockout, until after the ratification results are known.

It also orders both sides not to speak to the media after issuing the single joint statement.

This report by The Canadian Press was first published July 31, 2023.