Supreme Court explains why it invalidated Terrebonne results in 2025 federal election

Liberal candidate Tatiana Auguste, right, speaks to supporters with from left to right Minister of Health Marjorie Michel and Government Transformation, Public Works and Procurement Minister Joel Lightbound at her party's federal byelection night gathering in Terrebonne, Que., on Tuesday, April 14, 2026. THE CANADIAN PRESS/Christinne Muschi

MONTREAL — The Supreme Court of Canada says it decided to invalidate a federal Liberal candidate’s one-vote victory in the 2025 federal election to ensure public trust in the electoral process.

The court says the choice to annul the election result ensures that a federal election is not decided by a known and correctable administrative error.

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

• READ MORE: Supreme Court invalidates result from spring federal election in Terrebonne riding

The Bloc candidate challenged the election results 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.

A Superior Court judge rejected Sinclair-Desgagné’s request for a new election, but the Supreme Court reversed that ruling in a split decision delivered from the bench in February.

Justice Mahmud Jamal wrote for the majority that 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 error.

Auguste went on to win the riding for a second time in a byelection in April.

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

Morgan Lowrie, The Canadian Press