In announcing his resignation, McGuinty will end some 23 years as the Liberal representative for Ottawa South.
McGuinty stepped down as Ontario premier earlier this year after nine years on the job and 16 years 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.
McGuinty remains a central figure in Ontario's gas plant scandal. The province's privacy commissioner last week said senior staff members in both McGuinty's office and that of Ontario's energy minister intentionally deleted emails about the cancellation of the gas plants in Oakville and Mississauga.
The commissioner's report added fuel to opposition accusations that Ontario's Liberal government tried to cover up the full cost of cancelling the controversial projects.
Word of his impending resignation as an MPP came shortly after his successor, Ontario Premier Kathleen Wynne, avoided a summer election by winning NDP support for the province's new budget.
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