"This is a historic day that our members worked very hard for", said Jerry Dias, Unifor National President.
The BC Liberals failed to clinch a majority when they lost a crucial district to the NDP on 24 May, a fortnight after the 9 May election, after all absentee ballots were tallied.
The Green Party says it will support the NDP for four years, in order to ensure a stable government. Horgan said there was no discussion about having his party absorb Weaver and Green MLA-elects, Sonia Furstenau and Adam Olsen into the NDP.
Unifor is Canada's largest union in the private sector, representing more than 310,000 workers.
The leaders of the BC Greens and the BC NDP intend to form a minority government.
The final count left the Liberals one seat short of a majority with 43 seats to 41 for the NDP and three for the Greens.
Weaver said the deal comes after four sessions with the NDP, which will ratify an agreement with its caucus on Tuesday.
The Greens' longstanding opposition to new and expanded oil pipelines has been well-documented, and that stance appears to have played an integral role in their negotiations with the NDP, who have also opposed projects like Kinder Morgan's Trans Mountain expansion, which has already been approved by the federal Liberals.
Over the weekend, Weaver was seen watching a rugby match with BC NDP Leader John Horgan in Victoria, although the pair claimed they didn't talk about politics.
B.C. Liberal Leader Christy Clark. JONATHAN HAYWARD THE CANADIAN PRESS
"We specifically did not ask for there to be a coalition", said Weaver.
"It's vitally important that British Columbians see the specific details of the agreement announced today by the BC NDP and Green Party leaders, which could have far-reaching consequences for our province", Clark said in a statement.
Alberta premier Rachel Notley (who had yet to comment on the announcement at the time of publication) delivered a strongly worded message at a news conference earlier this month in light of uncertainty surrounding B.C.'s election results and the fate of the Kinder Morgan pipeline.
"We can have a legislature where all MLAs can put forward good ideas that help people, and come together to support them", he said.
Horgan and Weaver have had a hard personal relationship in the past, with both parties heavily criticizing the character of the other leader.
By convention, Clark remains premier until she resigns or chooses to call the legislature back and test the confidence of the house with a throne speech and budget - even if her opponents have lined up against her.
"In the end we had to make a hard decision, a decision that we felt was in the best interests of British Columbia today".
They still think there's a chance the new B.C. government will choose to deal with more pressing campaign promises, such as political financing and electoral reform, rather than provoking a sustained battle that would end with certain defeat in the Supreme Court. Next morning, Weaver cancelled a scheduled follow-up session and made a decision to go with the more substantive offer from the NDP instead.
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