The drinking water in the city isn't safe to drink, the official said.
Public health officials are meeting Thursday morning to discuss the matter.
The information comes a day after the city issued a statement saying the water was safe to drink.
The Marie-Victorin school board issued a notice to schools in Longueuil — including those in Lemoyne, Saint-Hubert and Greenfield Park — telling staff and students not to drink the water.
Several residents have complained on the City of Longueuil's Facebook page that their drinking water smelled like oil.
The spill was caused by a generator at the Longueuil waste-water treatment plant on Wednesday morning, causing 28,000 litres of diesel to spill into the St. Lawrence River.
Longueuil officials did not return a request for comment Thursday morning.
Mayor Caroline St-Hilaire has called a news conference for 11:30 a.m. ET.