Dalton McGuinty officially resigned today as MPP for Ottawa-South, with Ontario Premier Kathleen Wynne defending his legacy and saying he should not be defined by the gas plant cancellation scandal that sped his exit from politics.

In a statement a day after word emerged of McGuinty's planned resignation, he thanked his family, friends and colleagues, and said leaving politics came at an "opportune time" with the end of the session in the provincial legislature.

"I leave politics with my idealism intact and a deep sense of gratitude for the opportunity to have served in public life," said McGuinty on Wednesday.

The former premier departs Queen’s Park  after nine years on the job and 16 as leader of the Ontario Liberals. He led the Liberals to two majority governments — in 2003 and 2007 — but was left with a minority government after the 2011 election.

Wynne, while speaking on CBC's Metro Morning, said McGuinty's legacy should not be defined by the recent scandal, and her minority Liberal government is "building on the foundation" laid by her predecessor.

"He is leaving after having done years of service to the people of Ontario," Wynne told host Matt Galloway. "I’m proud to have been part of his government."

But in an interview with CBC Kitchener-Waterloo, Wynne said McGuinty's departure does not necessarily mean he can't be called before the gas plant committee for a second time to testify.

McGuinty is a central figure in the Liberals' decision to cancel two gas plant projects, one in Oakville and another in Mississauga. They have cost taxpayers an estimated $575 million.

"I encourage anyone who is being asked to come and speak at the committee that they do that, and that certainly has been my advice to Dalton and any of his staff," she said.

McGuinty had only shown up in the legislature twice since he was replaced by Wynne in February — both times for confidence votes — but did not attend Tuesday's vote on the minority government's budget, which passed with support from the New Democrats.

McGuinty said the Ontario Liberals called a nomination meeting to elect a new candidate for Ottawa South late last month and a vote was scheduled for June 20.

Wynne working with privacy commissioner

Earlier this month, Ontario’s privacy commissioner slammed the Liberals in a report that found senior staffers in McGuinty's office intentionally deleted emails about the gas plant decision. The move to delete emails is now the focus of a criminal investigation. The police probe appears to have prompted McGuinty to move up his departure date; he had previously said he would continue to represent Ottawa-South until the next election.

Wynne repeated her claim Wednesday that "mistakes were made" in how the gas plant decision was handled.

"I’m not denying that there are issues that we’re confronting, issues that [McGuinty] confronted over the last year," said Wynne.

"We have to recognize there were decisions that were made about the process, around documents that I think shouldn’t have been made. I don’t know whether we’ll ever get to the point where we can pinpoint a moment when a particular decision was made."

Galloway asked Wynne if the move to delete emails pertinent to the gas plant decision amounts to a coverup by the Liberals.

"I think that people were acting in ways that they had been told to act," said Wynne. "I’m not excusing the mistakes that were made; documents should have been retained. The former premier has said that: That 'there wasn’t enough rigour in his office.'"

Wynne said her office is working with the privacy commissioner to ensure electronic documents and emails aren’t again illegally deleted to spare the government from scrutiny.

McGuinty's resignation means Wynne will now have to call three byelections. She must call byelections by mid-August in Windsor and London to replace former finance minister Dwight Duncan and former energy minister Chris Bentley — both resigned after Wynne won the Liberal leadership in January.

With files from The Canadian Press