CALGARY—TransCanada Corp. is negotiating the sale of a 49.3 per cent interest in the Iroquois natural gas transmission system that connects its main Canadian pipeline system to markets in the U.S. Northeast, including New York City.

The Calgary-based company says it has offered to sell the Iroquois system to TC PipeLines, LP, an affiliated limited partnership.

TransCanada would also sell its remaining 11.8 per cent interest in the Portland Natural Gas Transmission System, which is an interstate system that carries gas from Canada to New England, to TC PipeLines.

It previously sold a 49.9 per cent interest in the Portland system in January for $228 million (U.S.) to TC PipeLines.

TransCanada indirectly owns about 27 per cent of TC Pipelines. Both said in separate announcements Monday that terms of the sale weren’t being disclosed. The proposed deal is subject to negotiation of satisfactory terms and regulatory approvals.

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