Canadian regulators are expected to announce Thursday whether they will allow the reversal of a pipeline that could bring oil sands from Alberta to a refinery in Montreal.

The proposed reversal of more than 500 miles of Enbridge Inc.’s Line 9 between Westover, Ontario, and Montreal has drawn intense criticism from environmental groups in Canada and the United States. Environmentalists say the reversal is part of a longer-term plan to export oil sands from Alberta through ports along the eastern coast of Canada and the United States, including through a Canadian-owned oil terminal in South Portland.

No plans have been presented to export the so-called tar sands oil through South Portland, although the city is engaged in a long-running debate about possible waterfront zoning rules to prevent any flow of the crude through the city.

The Canadian Energy Board said in a prepared statement that it will release the full decision on its website at 4:30 p.m. Thursday.

The Enbridge pipeline, built in the 1970s, originally flowed west to east, but was reversed in the 1990s to pump oil from east to west and accommodate market demand. The proposal by Enbridge would return the pipeline to its original flow.

Portland Pipe Line Corp. operates a 236-mile underground pipeline that pumps crude oil from tankers in South Portland to the refinery in Montreal that’s connected to the Enbridge line. Company officials have said, since withdrawing plans to reverse the pipeline in 2008, that no project exists to export oil from Canada through Maine.

“Portland Pipe Line Corporation’s previously stated position that there is no project to reverse the flow of its pipeline proposed, pending or imminent is unaffected by announcements regarding other Canadian pipelines,” said Jim Merrill, a spokesman for Portland Pipe Line.

Environmental groups remain skeptical of the company’s assurances, and say the Canadian government is likely to approve the reversed flow from Ontario to Montreal, bringing oil sands products to New England’s door.

“It’s very unlikely that it won’t be approved,” said Adam Scott, climate and energy program manager for Toronto-based Environmental Defence.

Scott said that the current regulatory climate in Canada is favorable to petroleum companies, and that the decision’s effects will likely be felt in South Portland, where a City Council-appointed committee is drafting an ordinance that would ban exports of oil sands from South Portland.

“I think it ups the stakes a little bit in that it makes that project seem more real to people,” Scott said. “Our response (on Thursday) will look closely at whether the regulatory process actually listened to the thousands of voices that opposed this project.”

Since World War II, Portland Pipe Line Corp. has pumped crude oil to refineries in Montreal. Demand for that service has slowed in recent years, and the rise in the global price of oil has turned once-marginal sources of crude into opportunities. Those sources include 170 billion proven barrels contained in Alberta’s oil sands.

Critics say the crude, a combination of sticky bitumen, sand and water, is dirtier to refine and is far more dangerous to ship through pipelines, posing serious health, environmental and public safety hazards if spilled.

Oil companies, including Portland Pipe Line, say oil sands are no riskier to pump through a pipeline than traditional crude.

The Canadian regulators’ decision will renew pressure on South Portland, where voters in November narrowly defeated a divisive citizen-initiated ballot question that would have prevented any future flow of tar sands oil through the city.

Petroleum industry advocates argued that the ordinance would have hamstrung waterfront businesses unrelated to oil handling and prevented the existing oil industry from upgrading its equipment and implementing the newest safety measures.

The close vote sparked a six-month moratorium on new oil export projects in South Portland and led to the formation of a committee to propose an ordinance that would keep tar sands oil out of the city without harming waterfront businesses that do not handle petroleum.

While that committee continues to work, advocates for the petroleum industry have continued a quiet campaign to soften the image of its products and services. An industry-backed group most recently ran a newspaper ad expounding the virtues of Canadian oil sands.

“It’s just oil. From Canada,” read the Feb. 28 ad, sponsored by Energy Citizens, in the South Portland-Cape Elizabeth Sentry newspaper. The ad says that oil sands, once processed and diluted, flow through pipelines like any other petroleum that moves through pipelines.

Energy Citizens is supported by the American Petroleum Institute, the lobbying group for American oil and gas interests that heavily supported the campaign to defeat the South Portland ordinance in November.

Calls to the American Petroleum Institute were not returned Wednesday.

Matt Byrne can be contacted at 791-6303 or at:

mbyrne@pressherald.com

Twitter: MattByrnePPH