Canada’s top court says Elections Canada committed a serious administrative error capable of undermining the integrity of the electoral process when it failed to address problems with mail-in ballots in a Montreal-area riding ahead of the 2025 federal election.

The Supreme Court of Canada invalidated the Terrebonne riding’s election results in a February ruling and released the reasons for its decision on Friday.

The court said its decision to order a new election “enhances public trust in the electoral process by affirming that a federal election should not be decided by known and correctable official errors,” read the decision.

Liberal Tatiana Auguste was initially declared the winner in the Montreal-area riding of Terrebonne by one vote over Bloc Québécois candidate Nathalie Sinclair-Desgagné in April 2025.

The Bloc candidate challenged this result and demanded a byelection after a Bloc Québécois supporter complained that she had tried to vote by mail using a special ballot that was never counted.

The voter, Emmanuelle Bossé, said her special ballot was returned to her own mailbox due to an error on the postal code printed on a label supplied by Elections Canada.

A Superior Court judge rejected Sinclair-Desgagné’s request for a new election, but the Supreme Court reversed that ruling in a split decision, triggering a byelection that Auguste won on April 13.

Writing for the majority, Justice Mahmud Jamal said an Elections Canada employee had known for weeks that some voters had been sent envelopes with incorrect postal codes, but the agency did not take steps to correct the mistake.

“Elections Canada did not receive or count an elector’s vote because it took insufficient steps to correct the error, despite knowing that it had made a mistake, and despite its expectation that affected electors would have been contacted by one of its employees,” the decision read. “Elections Canada caused the error and had the ability and responsibility to correct it.”

The court heard that an Elections Canada employee had sent out incorrectly-addressed envelopes for mail-in ballots to about 40 electors.

He realized his mistake more than three weeks before voting day but did not inform any supervisors. The employee believed the mistake didn’t seem to be causing any problem “because the Terrebonne polling station was receiving completed special ballots in the ordinary course, including in envelopes bearing the postal code error,” the decision read.

Jamal wrote that the Superior Court erred when it concluded that an unintentional administrative error wasn’t serious enough to be considered a voting irregularity.

“Unintentional, honest, or good faith errors can also undermine the integrity of the electoral process if they prevent electors from exercising their right to vote,” the judge wrote.

Elections Canada said in a statement that it conducted a review of its special ballot process after the election, and has already made changes. The agency said it would carefully review the Supreme Court decision before deciding what further actions to take.

“Elections Canada is also always looking at new ways to better equip the tens of thousands of workers who make the delivery of federal elections possible,” the agency said in a news release.

The decision was split 6-3.

Writing for the minority, Justices Andromache Karakatsanis and Sheilah L. Martin said the clerical mistake was not serious enough to meet the high bar to justify setting aside an election.

They also warned that the decision could open the door to further challenges.

“If this mistake affecting one vote is enough to reopen an election result, it is hard to see what kind of error would not meet the threshold,” they wrote. “Such a low threshold would put all election results at risk, undermining the stability, certainty and finality of election results.”

The majority, on the other hand, wrote that the criteria for overturning an election remain stringent, and the decision will not “open floodgates” to electoral challenges for every human error made during an election.

This report by The Canadian Press was first published Sept. 18, 2026.