A major real estate developer wants to build a 750-foot skyscraper in downtown Boston, but the proposal will need some very special treatment from lawmakers before it sees the light of day, or in this case, blocks it out.

Mayor Marty Walsh is ready to sell the derelict Winthrop Square parking garage to Millennium Partners, the group that filled the old Filene's site with a towering skyscraper, for a cool $153 million the city's coffers and taxpayers would welcome.

A 55-story tower at the site would violate laws restricting what hours a building can cast a shadow onto Boston Common and the Public Garden.

Walsh's office says a full analysis of environmental impacts and mitigation is taking place, but advocates and local officials, warn that shadows could be ruining one of the key reasons people live in Boston in the first place

"The shadow is something that we as a city can control and given that these legacy parks are so important to the quality of life in the city, that they make the city livable, that as we add more development and more residents in the area we need to be all the more concerned and protective of these two parks," Elizabeth Vizza, executive director of civic group the Friends of the Public Garden told WGBH News.

More shadows and less lights impacts the horticulture and dew points in the parks and affects the plant life and human experience, Vizza said.

A representative from Millennium Partners did not return a request for comment.

This is the first time park conservation activists have gone up against developers and City Hall over shadows on Boston's parks. Advocates successfully fought Mayor Kevin White over a plan to redevelop much of Park Plaza into shadow-casting buildings in the 1970s.

The City Council, State Legislature and the governor would have to sign off on an exemption to the shadow laws for the tower. Local elected officials say they want a comprehensive review of the law and development rules instead of just pushing through an exemption for a tower project they've just heard about.

"For a lot of us, this was something that the city was sitting on. I mean, they knew about this and never raised it with any of the elected officials that represent the affected area until about, well officially, a week ago," Rep. Byron Rushing said.

Rushing isn't exactly confident that the Winthrop project can get off the ground as soon as the mayor may wants, but he does think compromise is possible.

"It is not the best way if you want to work something out. You know, the sooner you come to describing the problem, the better. So I hope that they're not in a rush," Rushing said.

Rep. Jay Livingstone agrees that the Winthrop Square project should be used as a way to reexamine the shadow laws to come up with something city and state regulators, as well as developers, can rely on and abide by going forward. The Back Bay Democrat met with city officials and said the city's planning department should have paid more attention to the issue of shadows from a major development project.

"They said during the bidding process they collected no shadow data whatsoever and didn't give this any thought. And it's disappointing if that's the case," Livingstone said.

"It's disappointing that they went through an entire public process without figuring this out."

Vizza says the Friends of the Public Garden are not anti-development and is looking for a "win-win" with developers and conservationists.

"We don't want to thwart development, but as a community, as this generation of stewards of these parks and of the city, we need to balance that development with protection of the parks," Vizza said.