A downtown Toronto riding will be without representation at the Ontario Legislature until next spring, as Premier Kathleen Wynne has decided not to call a byelection to replace Glen Murray, who announced Monday that he's leaving politics.

Speaking to reporters on Monday, the premier said she decided not to send Toronto Centre voters to the polls in order to avoid the "significant costs" of a byelection.

"I think it is only responsible that we not, so close to an election, incur the cost of a byelection," Wynne said.

Environment Minister Murray announced Monday that he will resign his seat on Sept. 1. A general election is set for June 7, 2018.

"We're moving into that period, well within a year of a general election, and historically there has been a practice of when you get that close it's not necessarily responsible to call a byelection. So we won't be doing that," the premier said Monday.

Murray, who was first elected in Toronto Centre in 2010, is leaving politics to become executive director of the Pembina Institute, an environmental think-tank.

"Serving the people of Toronto Centre as their MPP has been a privilege and a humbling learning experience," Murray said in a statement.

Cabinet shuffle

While Murray's legislative seat will sit vacant for months, his cabinet post has already been filled.

Wynne announced that Newmarket-Aurora member of the legislature Chris Ballard is taking over for Murray as environment minister.

Speaking alongside the premier on Monday, Ballard said he's passionate about the environment and will approach the new job with urgency.

"The file is so critical and so important that we can't turn the heat down on this one," he said.

Ballard had previously served as minister of housing and minister responsible for the poverty reduction strategy, positions that will now be filled by Toronto member of the legislature Peter Milczyn.

Murray's departure comes as a surprise, as he had previously indicated he would run again in next year's election.

Former mayor

Murray has been a fierce defender of his environmental positions and under his tenure the ministry implemented an ambitious cap-and-trade program aimed at reducing greenhouse gas emissions.

A former mayor of Winnipeg, Murray was first elected to the Ontario legislature in 2007 and served as minister of research and innovation and minister of training, colleges and universities under then premier Dalton McGuinty.

After Wynne was elected premier, she gave him the portfolios of infrastructure and transportation.

Wynne said in a statement that Murray has always been guided by his passion.

"He is unrelenting in his advocacy for the issues he cares deeply about, dedicated to the people he represents in Toronto Centre and across the province…. There is no doubt that Glen's voice, candour and passion at the cabinet table will be missed."

She also dismissed suggestions that Murray's departure is related to the Liberals' electoral prospects, which polls suggest are hurting.

"I do not see this as a vote of non-confidence," she said. "I see this as an individual having to make a decision about his life, and he is a friend and I wish him well."

Murray said he has a "high degree of confidence" the Liberals will be re-elected. He just decided that he had hit his political "best before date," he said.

'Led by example'

Wynne also cheered Murray's advocacy for the LGBT community, which has included his election as the first openly gay mayor of a large Canadian city.

"Glen's career has courageously led by example, paving the way for people to be their true selves and become whatever they want to be," she wrote.

Wynne alluded to Murray's outspoken nature, saying she is glad as leader that her cabinet and caucus members say what they think.

"From my perspective there is no point in having a group of people who simply nod and smile when I put forward a plan or an idea," she said.

Murray said in his statement that the decision to leave was a difficult one that he made with the support of his partner, Rick.

"I have also always viewed my life and career as split up into distinct chapters and often in response to serious challenges," he wrote. "When confronted with a choice between the 'unthinkable' and the 'impossible,' I will always take on the impossible to stop the unthinkable."

Wynne also announced Monday that francophone affairs will become a stand-alone ministry. Marie-France Lalonde, the corrections minister, who is already responsible for the file, will become the minister of francophone affairs.