TORONTO -- Doug Ford's plan to open a large protected green space around the Toronto region to housing development would make the map of the area look like "Swiss cheese," Liberal Premier Kathleen Wynne said Tuesday, calling the Tory election pledge an environmental setback for the province.

With just over a week before the start of Ontario's election campaign, Wynne ratcheted up her attacks on the Progressive Conservative leader, calling his plan for the Greenbelt "absolutely wrongheaded."

"If you open up the Greenbelt and make it into a Swiss cheese map you never get that back," she said. "You never get that water protection back. You never get that agricultural land protection back."

The Greenbelt -- the world's largest permanently protected green space --is a 7,200-square-kilometre area that borders the Greater Golden Horseshoe region around Lake Ontario. It was protected from urban development by legislation in 2005.

On Monday, Ford said he supports the Greenbelt "in a big way," but would allow some development in the region to ease the housing crisis in the Greater Toronto Area.

His comments came after the Liberals accused him of making private deals with developers and pointed to an online video of him -- apparently taken in early February when he was a Tory leadership candidate -- promising to open up a "big chunk" of the protected region.

Wynne said opening up the Greenbelt for development is unnecessary.

"The fact is there is enough land in the Greater Toronto and Hamilton Area to build two more cities the size of the Mississauga," she said. "There is a lot of land that is available."

Ford has said that for every piece of the Greenbelt opened for development, he would add equivalent land to the protected area to ensure it doesn't change in size -- but he hasn't said how that would work.

Wynne acknowledged that some areas around the border of the Greenbelt have changed since the protected area it was established over a decade ago, but that was part of the original plan for the region, she said.

"He wants big chunks of the Greenbelt," she said of Ford's plan. "That means taking a big hole out of the middle of the Greenbelt, or many holes out the middle of the Greenbelt."

Tory Parliamentary Leader Vic Fedeli dismissed Wynne's remarks and said she was using Ford's comments on the Greenbelt to distract from other issues ahead of the June 7 election.

NDP Leader Andrea Horwath said Ford's plan is bad policy.

"Newsflash: farms feed cities. There is no reason whatsoever to even contemplate paving over the Greenbelt unless you're trying to make a good buck for your friends in the development industry," she said. "And that's not what public policy, government and public decision-making should be all about."