A high-speed rail corridor between Toronto and Windsor, Ont., is in the works -- an idea floated in Ontario for decades, but the premier says this time it's happening.

A government-commissioned report looking at the feasibility of such a project pegs the cost around $20 billion and suggests looking to the private sector for opportunities to partner on funding.

Preliminary design work and a $15-million environmental assessment are beginning, with an aim of full service in 2031. The government hopes that by 2041 the rail line would see 10 million users annually and take more than five million cars off southwestern Ontario highways.

Trains on the planned rail link would travel up to 250 kilometres per hour, which is expected to cut travel times between Toronto and Windsor from four hours to two.

The government says Ontario is believed to be the first province to undertake a "rail transformation" on this scale. Canada is the only G8 country that doesn't have a high-speed rail system under construction or in operation, according to the government-commissioned report.

Premier Kathleen Wynne said not only will the high-speed rail link save people time, but it will serve as an economic driver along a corridor that is home to more than seven million people and over 60 per cent of Ontario's economic activity.

High-speed rail has been debated since the 1970s, with excuses that the country is too large, the population is too small, and it's not worth it, but it needs to happen now, Wynne said.

"We have lost opportunities as a province because we haven't had this line in place," she said Friday in London, Ont., a mid-point for the rail route.

"I get that there's going to be a robust conversation about how much it will cost -- how much the project will cost, how much people will pay -- we obviously are going to work to keep the costs down in every way possible, but we've got to move ahead. We've got to not let this be another lost moment. We've got to do it this time, folks."

Ontario's Progressive Conservative transportation critic dismissed the announcement as a re-election ploy.

"Southwestern Ontario has been waiting on high-speed rail for years," Michael Harris said in a statement. "Now, a year out from the election, the Wynne Liberals have shown up in London to re-announce another transit project they will never deliver on."

Provincial New Democrats representing southwestern Ontario ridings issued a joint statement saying people in their communities "are wondering today whether this is just another hollow election promise."

In 2015, the government tapped a special adviser on high-speed rail to assess the possibility of such a project, and David Collenette concluded there is a business case for it. It would alleviate pressure on the heavily-travelled Highway 401, free up air travel capacity by lessening the need for short-haul flights and spur regional development, he wrote.

Collenette, a former federal transportation minister, recommended the first phase connect Toronto and London by 2025, then extend the line to Windsor in a second phase.

The train would travel between Toronto's Union Station and Pearson International Airport, Guelph, Kitchener, London, Chatham and Windsor.

Collenette looked at the possibility of a line operating mostly on a dedicated right-of-way with top speeds to 300 kilometres per hour, but found it would have been more expensive than a line operating on a mixed-use railway.

Capital costs are not generally fully recoverable through fares, but international high-speed rail systems have typically seen revenues cover operating and maintenance costs, Collenette wrote.

A new public entity will be established to oversee the project.