The resignation of another Ontario cabinet minister Saturday fuelled speculation that the race for the provincial Liberal leadership may be heating up, and prompted a minor shakeup in the ranks of the minority government.

Toronto-Centre MPP Glen Murray broke the news in a Twitter message Saturday morning that he resigned as the province's minister of training, colleges and universities — a necessary step for those seeking to replace Premier Dalton McGuinty as party leader.

He called it a "difficult decision" and said he would release more details at a "special announcement" Sunday.

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A spokeswoman for Mr. Murray would not discuss his plans, saying the MPP would lay out his intentions at the event.

Mr. Murray's decision led the Liberals to rejig their lineup Saturday, with Mr. McGuinty announcing MPP John Milloy will take over the training portfolio on top of his role as minister of community and social services. The change will come into effect early next week.

Mr. Murray, who served as the mayor of Winnipeg from 1998 to 2004, was first elected to the Ontario legislature in a 2010 byelection.

His win maintained the Liberal's grip on the downtown riding, which has been a stronghold for the party since George Smitherman was first elected in 1999. Mr. Smitherman left the post in 2010 to enter the Toronto mayoral race, which he lost to Rob Ford.

Mr. Murray was re-elected in 2011 and given the cabinet position in the Liberals' minority government.

Several ministers have expressed interest in replacing Mr. McGuinty following his surprise resignation last month after nine years as Premier and 16 years as leader of the Ontario Liberal party.

The Premier confirmed former Municipal Affairs and Aboriginal Affairs Minister Kathleen Wynne would throw her hat in the ring after she stepped down from cabinet this week, but no official announcement was expected before Monday.

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Ms. Wynne, 59, was first elected in 2003 and has also been in charge of the high-profile education and transportation portfolios.

Other potential candidates said to be considering a bid include former cabinet minister Sandra Pupatello and ex-MP Gerard Kennedy.

Finance Minister Dwight Duncan, Energy Minister Chris Bentley, Education Minister Laurel Broten and Economic Development Minister Brad Duguid have all decided not to run.

Those interested in the job must submit their names by Nov. 23. The fee to enter is $50,000 and candidates can spend up to $500,000 on their campaign.