The pipeline operator had envisioned the Energy East and Eastern Mainline projects to help increase western Canadian oil producers' access to east-coast and offshore refineries and increase natural-gas supplies to Southern Ontario.
The Calgary-based company refused to give further explanation.
But as prices tumbled and regulatory hurdles aimed at protecting the environment multiplied, the business case for Energy East became weaker.
The Energy East pipeline would have carried oil from Alberta and Saskatchewan to New Brunswick, and would have seen 612 kilometres of natural-gas pipeline in the province converted to transport 1.1 million barrels of crude oil a day to the Maritimes.
October 29, 2014: A report released by environmental groups questions whether Energy East is necessary to supplant Eastern Canada's oil imports from the foreign suppliers frequently mentioned by TransCanada.
In the summer of 2016, the National Energy Board's review of Energy East was compromised after it was revealed by the National Observer that former Quebec Premier Jean Charest met the chairman and two commissioners on the National Energy Board while working for TransCanada.
The Canadian Energy Pipeline Association says it is "extremely disappointed with the decision".
As a result of the decision, TransCanada expects an estimated $1 billion after-tax, non-cash charge to be recorded in the fourth quarter, the company said Thursday.
When it was announced in 2013, Energy East was hailed as a nation-building project that would unlock Canadian exports and boost economic growth along its route.
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Energy East would have given oil producers in Alberta and Saskatchewan, who are heavily dependent on buyers in the USA, another market for their crude by carrying about 1.1 million barrels a day to refineries and a marine-shipping terminal in eastern Canada. "We still believe that", Gallant said.
The company said in a statement that "after a careful review of change circumstances", it would tell Canadian energy regulators and those in Quebec it was no longer pursuing the project.
Trans Mountain and Line 3 together represent more than $11.6 billion in investment that will support "thousands" of jobs, Carr said.
Cenovus Energy Inc., an oil-sands producer that had contracted to ship 200,000 barrels a day on Energy East, said the cancellation will reduce options for Canada's oil producers to reach global markets.
"We still believe that", Gallant said.
"The abandonment of the Energy East project is a major victory for the municipal world", Coderre wrote.
"The Trudeau government is going to have to take a real hard look at whether their own heavy-handedness with that policy also led to this decision".
Quebec politicians, along with Indigenous and environmental groups, welcomed the project's demise, branding it as a harbinger of the inevitable death of fossil fuels and a reminder of the need for further green energy development.
On Thursday, Notley called on the NEB to explain what future project reviews will look like in Canada. The company warned then that it could cancel the proposed 2,800 mile oil pipeline from Hardisty, Alberta, to Saint John, New Brunswick, which would have been most expensive project in TransCanada's history.