Trans Mountain Corp, which is owned by the Canadian government, last month applied for a variance on a section of pipeline between Hope and Chilliwack, British Columbia, after encountering "very challenging" construction conditions due to the hardness of the rock it needs to drill through.

The company wants to install a 30-inch-diameter (76 cm) pipe instead of a 36-inch (91 cm) pipe as planned, which would shorten the installation schedule by approximately 55-60 days.

In a letter dated Nov. 23, the CER said it requires "further information or justification" regarding Trans Mountain's submissions so far, and will hold an oral hearing in Calgary on Monday.

Trans Mountain had asked the regulator to make a decision on the variance request by Nov. 30 to "advance the timely completion" of the expansion project.

TMX will ship an extra 590,000 barrels per day of crude from Alberta to Canada's Pacific Coast. The C$30.9 billion project has been plagued by cost overruns and regulatory delays.

The expanded pipeline is scheduled to start operating late in the first quarter of 2024, and any further construction hold-ups risk delaying the start date.

Earlier this month TMX was ordered to stop work for nearly two weeks after regulators found several environmental non-compliance issues in a wetland area near Abbotsford, British Columbia.

In September, the CER approved a route deviation request from TMX for a section of pipeline near Kamloops, despite opposition from local Indigenous groups.

The CER said no other parties had expressed concern about TMX's latest variance request.

(Reporting by Nia Williams; Editing by Mark Porter and Leslie Adler)

By Nia Williams