The dream of Canada's first national urban park is suddenly falling apart as Ontario is pulling back its portion of the land in a protracted dispute with Ottawa over environmental protection.

Ottawa announced the proposed Rouge National Urban Park with fanfare in the 2012 budget. The idea had broad political support, and veteran Toronto-area Tories such as Pauline Browes had long advocated for it.

Ottawa argues that the traditional environmental protections of a national park are not practical in a city, and that forcing farmers to meet the province's suggested standards for conservation would be unfair. The bill proposes protections that Ottawa says are stronger than provincial laws. However, several environmental groups sided with Ontario on Thursday, saying the bill fails to meet or exceed existing provincial laws and does not come up to international standards for conservation.

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Federal Environment Minister Leona Aglukkaq said Ontario would "prioritize nature over the conservation of culture and agriculture."

The long and narrow tract of land runs from Lake Ontario in the south to the Oak Ridges Moraine in the north, covering 5,665.6 hectares. Only about 2,023 hectares is federal land, and most of the northern third of the proposed park is agricultural property.

Supporters of the park say it would give urban Canadians in the Greater Toronto Area the chance to experience a national park, possibly for the first time.

Brad Duguid, the province's Minister of Economic Development, Employment and Infrastructure, said the province's main concern is Ottawa's refusal to accept its proposals for environmental protection. The province says its decision will remove about two thirds of the land from the national park, although it insists the land would remain protected under provincial law. Ottawa says the provincial share is not that high.

Mr. Duguid announced Ontario's position on Thursday after a Conservative-dominated committee in the Senate rejected amendments to a federal government bill aimed at creating the park that would have added Ontario's preferred wording on environmental protection.

In an interview, Mr. Duguid made clear that voters should resolve the issue.

"There's a federal election this year. I expect that following that, whether this government's re-elected or there's a new government elected, there may well be a change of heart by then," he said. "We have all opposition parties supportive of our position on this. There's every reason to be optimistic that the national park will take place. It's just unfortunate that the Harper government's decided to blow an opportunity to actually make it happen."

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Some view the comments as evidence Ontario's decision is primarily motivated by election-year politics.

Glenn De Baeremaeker, a Scarborough city councillor who drew up the boundaries of the park more than 25 years ago as an environmental advocate, said provincial concerns about the environment are "make-believe" objections.

"When you look at everything the federal government is doing, it is better than what we have had in the Rouge watershed ever since I've been involved," he said. "I believe, for partisan reasons only, the Liberal government in Ontario is saying, 'We don't want a Conservative federal government looking good,'" he said.

In 2012, the federal government announced $140-million over 10 years toward creating the park.

Ms. Aglukkaq is responsible for Bill C-40, the federal legislation that the House of Commons supported last month over the objections of the opposition NDP and Liberals.

Ms. Aglukkaq and Mr. Duguid spoke on Wednesday before the Senate committee vote, but were unable to reach a compromise.

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"Your approach thus far has called into question your sincerity with respect to working together constructively to find a solution," Ms. Aglukkaq wrote in a March 11 letter that was copied to the Ontario Premier. "I am not interested in playing political games at the expense of farmers and the environment."

Scarborough-Rouge River NDP MP Rathika Sitsabaiesan and her NDP colleagues initially supported the government bill, but later switched positions after hearing from stakeholders. Ms. Sitsabaiesan said Ottawa could have avoided the current outcome by accepting the amendments.

"When the Conservatives push it through, the new Rouge National Urban Park is not going to include the current Rouge Park. Does that make any sense?" she said. "I don't want politicians to be the local heroes. I want the hero to be the park and the people who fought for the park."