Almost the entire city of Montreal will remain under a boil water advisory until at least midday Thursday, after tests at a city filtration plant showed the water was not safe to drink.

City officials advised residents Wednesday morning that the drinking water may not be safe for consumption everywhere south of Highway 40, also called the Metropolitan Expressway.

That includes the neighbourhoods of Mount Royal, Westmount, Cote Saint Luc, NDG, Montreal East and Verdun. Anjou, north of the Metropolitan, is also under the boil advisory.

In all, approximately 1.3 million residents are being advised to bring their water to a rolling boil for 60 seconds before drinking it or using it for cooking, washing food, or brushing teeth. Non-boiled water can be used for washing clothes or for bathing.

The advisory comes after a failed test result at the Atwater filtration plant. The plant is the country’s second-biggest filtration plant and is currently in its fourth year of extensive maintenance work.

Valerie De Gagne, a spokesperson for Montreal, described the advisory as unprecedented.

"To my knowledge, it’s the first time that we have in Montreal a boil-water advisory for so many people," she told The Canadian Press on Wednesday.

CTV Montreal’s Rob Lurie says it appears that during the maintenance, water levels in the plant were lowered and that’s when a potential problem was discovered.

"It was so low that they discovered some sediment in the water early this morning and for that reason, they’re taking this precautionary measure," Lurie explained.

Montreal Fire Department Division Chief Gordon Routley says the advisory is purely a precautionary measure and it’s unlikely the water is contaminated, though they are still waiting for test results.

"We don’t think there’s any bacterial contamination in the water," he told CTV.

"What was noticed was turbidity -- discolourization and particulate matter in the water. Just to be sure, the boil water advisory has been put in place," he said.

A number of Montreal residents on Wednesday reported seeing brownish coloured water gushing from their taps.

"When I first started running (the water), it was kind of a muddy brown colour," Kelly Albert, a resident of Verdun told CTV Montreal.

Albert said she immediately went to the store to purchase bottles of water. “The water was quite cloudy this morning so I didn’t want to take a chance,” she said.

Albert was not alone. Many Montreal residents rushed to stores, filling their shopping carts with cases of bottled water. At Wal Mart, shoppers were out of luck as the giant retailer ran out of bottled water, reports CTV Montreal.

At local schools, teachers were advised to close off their drinking fountains. "Basically we were told to make sure that all the water fountains were covered and blocked off," vice-principal of Pierre de Coubertain Elementary Sonia Marotta said.

She said the school had cases of bottled water on site which they keep in case of emergencies.

The advisory also prompted some hospitals to shut off their water fountains.

At the McGill University Health Centre, the cafeteria stopped using tap water while staff members handed out bottled water to patients.

Some local businesses also took precautionary measures. All Starbucks in the affected boroughs stopped selling coffee and tea. Only pre-bottled beverages and pastries were being sold on Wednesday.

And at a local bakery, staff were using boiled water since the advisory was issued Wednesday morning. "A lot of baked goods, they use a lot of water with flour so we don’t want to take a chance on that," Peter Tsatoumas, owner of New Navarino Bakery and Café said. "It’s a big inconvenience because of that."

Officials in Montreal said tests are being conducted to determine whether the water is, in fact, contaminated. "Once we get the results, we will be able to maintain the advisory or take it down," said De Gagne.

Officials have not yet said what caused the water levels at the plant to dip so low and whether the incident was a result of human error.

With files from CTV Montreal and The Canadian Press