Bob Rae, the Liberal MP and former interim leader of the party, will resign his Toronto Centre seat.
An emotional Rae, 64, told his Liberal colleagues of his decision at Wednesday's party caucus meeting. He had previously announced he was taking on a job as chief negotiator for several First Nations groups in northern Ontario who are negotiating with the provincial government over the Ring of Fire mining development.
Rae told reporters his work as a negotiator required too much of his time and made it impossible for him to continue as a member of Parliament.
"This is not about money, this is about time," Rae said at a media appearance Wednesday morning with Liberal Leader Justin Trudeau.
"It's been a very difficult decision and ... quite an emotional one for me," Rae said.
Rae was first elected as a Liberal MP in 2008 after he ran for the Grit leadership in 2006, finishing third. He assumed the interim leadership in 2011 after the defeat and resignation of Michael Ignatieff.
Rae was previously a New Democratic Member of Parliament in the late 70s and early 80s. He moved to provincial politics and quickly became leader of the New Democratic Party of Ontario in 1982.
In 1990, he became the first NDP premier of Ontario.
On Wednesday, Rae said he had an excellent relationship with Trudeau "that has only grown stronger over the last weeks and months.”
He said he was "more than confident Mr. Trudeau will become prime minister of Canada,” adding one of his big regrets is that he won't be there as an advisor for the "next leg of the journey."
Rae also categorically ruled out a run for mayor of Toronto.
“I have a marriage and some conditions have been established. I will not be a candidate for the mayor of Toronto,” he said.
Rae was the scion of a noted family. His father, Saul Rae, was a highly respected Canadian diplomat and took the family to postings that included Washington and Geneva.
The younger Rae graduated with honours from the University of Toronto, where he also later received a law degree.
For a time, he roomed with Michael Ignatieff, who later would be a rival for the Liberal leadership.
His university record brought him a Rhodes Scholarship to Oxford University.
He has said his first foray into politics was as a volunteer during Pierre Trudeau's legendary first campaign in 1968, when he was 19.
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Bob Rae Resigns - Twitter Reacts
BOB RAE'S STATEMENT
Some months ago I agreed to work with the Matawa Tribal Council in northern Ontario as their negotiator in dealings with the government of Ontario.
The Ring of Fire mining development will have a huge impact on the communities in the area and well beyond. How positive that impact could be has yet to be determined, and will depend on the outcome of the discussions that are now underway, and will only intensify in the time ahead.
It has become clear to me that the full scope of the negotiator's job is no longer compatible with my also serving as a member of Parliament. And so I face a choice.
I have decided to return to my profession as a lawyer and mediator, to continue working for the Matawa Tribal Council, and to step down as the Member of Parliament for Toronto Centre.
This has been a difficult personal decision. I was first elected to Parliament in 1978, and was deeply honoured to have had the chance to serve again these past five years, as well as to lead the Liberal Party at a time of change and renewal. I have made wonderful friendships, and am so proud of the renewal and rebuilding that has been accomplished over the last few years. I'm leaving the party in confidence that the party is in good shape.
I am especially happy that as interim leader I was able to champion aboriginal issues, and share the importance of mental health with my colleagues. I also particularly enjoyed having the confidence of Stephane Dion, Michael Ignatieff and Justin Trudeau as foreign affairs spokesman for the Liberal Party. I shall miss this work very much.
I want to thank my constituents in Toronto Centre, my colleagues and friends in the Liberal Party and all parties in Parliament, my leader, Justin Trudeau, and the people of Canada for giving me the chance to serve. It has been an honour and a pleasure.
Helping to improve the life of First Nations people has been a longstanding commitment of mine, and this opportunity to serve is one I felt I could not decline.
I have told Mr. Trudeau and my colleagues that I shall continue to work for the Liberal Party, and I look forward to remaining engaged in Canadian public life.
With files from The Canadian Press