Montreal has begun dumping eight billion litres (2.1 billion gallons) of untreated sewage into the St Lawrence River, which forms part of the Canada-US border.

The operation is expected to last a week and has angered environmentalists who are worried about raw sewage in the water and the possibility of detritus such as condoms washing up on river banks.

Mayor Denis Coderre said the move was necessary so that workers could relocate a snow chute - a large opening to bring water from melting snow to a wastewater treatment plant.

He said: "Nobody is pleased with that situation but we have to do it and it's to enhance our own structure for the future and I think that's important."

Citizens are being asked not to flush certain items during the dumping period such as condoms, medications and tampons.

The discharge plan was originally timed for last month but it was suspended by Canada's previous Conservative government during the election campaign.

Canada's new environment minister, Catherine McKenna, said the city would have to meet certain conditions during the dump including increased monitoring of the river and a clean-up plan for affected areas. The mayor says the requirements will be met.

The dumping decision has angered US senator Charles Schumer, who has demanded US environmental regulators ask Canada to halt the move.

The US Environmental Protection Agency has no regulatory authority over Canada.

Mr Schumer said the river should be treated as a single ecosystem, because fish and birds move upstream and downstream, as do recreational and fishing boats.

He stressed that the US and Canada had worked successfully in the past to protect the region's waters through the Great Lakes Water Quality Agreement.

The river lies entirely within Canada and flows downstream of Montreal.

It flows from Lake Ontario northeast into the Gulf of Lawrence and borders the US for 114 miles (183km).