(Adds details, background)
Oct 19 (Reuters) - Canadian Oil Sands Ltd asked its shareholders to reject Suncor Energy Inc's offer, saying it "substantially undervalued" the company's ownership in Syncrude and was "entirely opportunistic".
Canadian Oil Sands, with a 37 percent stake, is the largest shareholder in Canada's biggest synthetic crude project, Syncrude, in which Suncor also has a 12 percent interest.
A steep fall in oil prices globally since June last year has driven consolidations in Canada's oil sands industry, which has one of the world's highest operating costs and lowest prices.
Suncor, Canada's largest oil producer, had offered to buy the company earlier this month, valuing it at C$4.3 billion ($3.33 billion) as it sought to expand in the country's oil sands.
Following the hostile offer, Canadian Oil Sands adopted a poison pill to thwart Suncor's hostile takeover bid.
In September, Suncor bought a tenth of the Fort Hills oil sands project in northern Alberta from French oil company Total . ($1 = 1.2920 Canadian dollars) (Reporting by Shubhankar Chakravorty in Bengaluru; Editing by Anil D'Silva)