The Supreme Court of Canada has annulled the 2025 federal election result in the Montreal-area riding of Terrebonne, ordering that a by-election be held after determining that ballot handling errors compromised the integrity of the vote.

Liberal candidate Tatiana Auguste was declared elected by a single vote following a judicial recount, defeating Bloc Québécois incumbent Nathalie Sinclair-Desgagné. The razor-thin margin triggered legal proceedings after Sinclair-Desgagné challenged the validity of the result, arguing that at least one mail-in ballot was not counted due to a postal code misprint and that broader administrative issues affected dozens of special ballots.

According to court records and submissions, Elections Canada acknowledged that a printing error involving a postal code led to the mishandling of special ballots intended for the riding. The agency indicated the problem affected at least 40 special ballots. A lower court previously characterized the situation as human error and concluded that it did not justify overturning the result.

In a unanimous ruling, the Supreme Court found that the errors were attributable to the electoral authority and not to voters and that the integrity of the electoral process required corrective action. The court held that maintaining public confidence in the administration of elections outweighed the narrow finality of a one-vote margin. The decision voids the previous declaration of election and compels a new vote in the riding.

Terrebonne, located north of Montreal, has historically been a competitive constituency, with the Bloc Québécois holding the seat for multiple terms prior to the 2025 contest. The annulment creates renewed political uncertainty in a closely divided House of Commons, where the governing Liberals remain short of a majority.

A writ for a by-election is expected to be issued in the coming weeks. Until a new member is elected, the seat will remain vacant.