Montreal Mayor Denis Coderre blasted the federal government Wednesday evening for being "irresponsible" by halting the city of Montreal's plans to to dump eight billion litres of raw sewage into the St. Lawrence River.

"This decision by the government is irresponsible and unfair," Coderre said. "It is against Montreal and it is against the St. Lawrence River...They have some political game — some puppet master to score political points."

Coderre's comments came about three hours after Canada's environment ministry released a statement. In it, Environment Minister Leona Aglukkaq said she has "instructed Environment Canada to immediately have an independent expert scientific review of all information related to this project conducted."

Conservative candidate Denis Lebel, Stephen Harper's Quebec lieutenant, announced the decision to put the Montreal sewage dump on hold at a news conference in Wednesday. (CBC) She added that an "independent scientific review by a third party will ensure the best possible protection for the St. Lawrence."

The statement said the sewage dump plan would be put on hold in the interim, citing Section 37 of the federal Fisheries Act.

Conservative candidate Denis Lebel, Stephen Harper's Quebec lieutenant, made the announcement on Aglukkaq's behalf this afternoon.

Coderre shot back, saying all the scientific information was sent to Environment Canada. He held up a printed email dated Oct. 10, 2015, from the regional director of Environment Canada.

Montreal Mayor Denis Coderre holds up an email from an Environment Canada official, which he said indicates that the ministry had all the scientific information necessary to reach a decision on the city's dump plan by Oct. 14. (CBC) "I have the email here and he's saying clearly that, 'I have all the information that I need. There is no problem. We can take all the decisions on the analysis given,'" Coderre said. "So they want to buy some time."

Late Wednesday, Environment Canada contacted CBC to say Coderre's interpretation of the email was incorrect.

"The official's comments in the email were to acknowledge that Environment Canada had received the additional information provided by the city, ... and that the information was sufficient for officials to proceed with their analysis, as opposed to the overall project," said Environment Canada spokesman Mark Johnson, in an email.

Coderre said the move by Ottawa is strictly a political one, adding that cities across Canada dump wastewater into the ocean and lakes every day and they have never been blocked by the feds.

"Is it because Montreal doesn't vote Conservative because they are playing politics on Montrealers' backs?" Coderre said. "Will we do the same thing for Victoria? For Toronto? For other cities where this is done? You can see this makes no sense."

Coderre said he will respect the law, and that the city's dump plan can be put off until Oct. 23.

Until then, he invited Environment Minister Aglukkaq to get a better understanding of the dossier.

"We can work hand in hand, they can come with us….but if she needs a crash course on the river — I'd like to know if she knows where the St. Lawrence River is — my officials are not there to play politics on Montrealers' backs. My officials are there to improve the quality of the water and the quality of life of Montrealers," Coderre said.

Mixed reaction from other politicians

Sylvain Ouellet, a member of Projet Montréal, the opposition at city hall, said he was pleased with the federal government's decision. The opposition party has been highly critical of the city's plan, despite Quebec's environment ministry giving the city approval.

"Since the beginning of the crisis ... the official opposition asked for some studies. There's lots of things that are still not clear for us and for the general public," said Ouellet. "We are very pleased that Environment Canada will ask for more information and will ask independent experts to see if they have some alternatives to either reduce the amount of the spill or probably remove some pollutants from the spill."

Sources within the provincial government told Radio-Canada that Quebec questions whether Ottawa is acting responsibly on this file.

The sources said the federal government has not issued a timeline or deadline, and that's a major concern given that experts advised the best time to dump the wastewater would be at the end of October, when the fish spawning period is over and the cooler water temperature limits the spread of bacteria found in sewage.

Sources said delaying the dump plan could have more serious consequences on the infrastructure and the environment.

Bloc Québécois Leaders Gilles Duceppe says Environment Canada, which has known about Montreal's sewage dump plan since September 2014, should have intervened sooner. (Radio-Canada) Bloc Québécois Leader Gilles Duceppe accused the federal government of dragging its feet on the issue — in Montreal and other cities across Canada.

"They are asking scientists to intervene — something they should have done a long time ago. I hope they will intervene quickly," Duceppe said. "And I'm asking Mr. Lebel to have the same attitude towards the City of Toronto, which dumps 10 billion litres of wastewater into Lake Ontario, and the City of Victoria which dumps 30 billion litres of wastewater into the Pacific Ocean every year."

'A political decision,' expert says

One wastewater expert called Lebel's announcement to halt the city's dump plan "disingenuous" and a political move.

Sarah Dorner is an associate professor of civil, geological and mining engineering at École Polytechnique, and the Canada Research Chair in source water protection

Sarah Dorner, the Canada Research Chair in source water protection, says the federal government's move to halt the city's sewage dump plan is political, and 'not based on any kind of scientific opinion.' (Radio-Canada) "I think they didn't want to deal with this before the election. This is a political decision. It's not based on any kind of scientific opinion. And I find it rather troubling that [Lebel] doesn't hold his own scientists of Environment Canada in higher regard on this matter," Dorner said.

Sarah Dorner, Canada Research Chair in source water protection, says Ottawa's move to halt Montreal's dump plan is not based on science. 8:10

"Honestly, this river is one of the most well-studied in the world and some of the best expertise in this area is actually housed within Environment Canada and so it is surprising that they would need an independent expert who would not know nearly as much about the ecosystem, about the hydrodynamics of everything relating to the St. Lawrence River...As a scientist, I have full confidence in the expertise of the scientists at Environment Canada. They're there to do their job. Why can't the minister?"

The City of Montreal wants to dump eight billion litres of sewage directly into the St. Lawrence over a week-long period, starting Sunday, a day before the federal election.

A major sewer interceptor must be drained to allow construction work to be completed near the Bonaventure Expressway, a raised highway that runs along the downtown shoreline which is being torn down.

Last week, Environment Canada stepped in, saying it wanted to analyze the file more closely.

About 90,000 people have signed a petition against the sewage dumping plan.