Saskatchewan province, home to Potash Corporation, has asked the Canadian government to block mining giant BHP Billiton's hostile bid for the fertiliser group.
The province's premier said the takeover was not in the interests of Saskatchewan or of Canada.
Last month, Potash asked a US court to block the bid.
The Anglo-Australian mining group offered $39bn (£25bn) for the firm in August.
"In the interests of jobs for Saskatchewan families, in the interest of quality of life that we prize that is funded by revenues to the government, we must say no to this hostile takeover," said the province's premier Brad Wall.
"This is not a normal market transaction or a normal takeover. There hasn't been a takeover in the history of takeovers, that we're aware of, that involves almost a third of the world's supply of something as strategic as potash."
The Canadian government must now decide whether to block the deal.
Canadian Prime Minister Stephen Harper has said his government could block a takeover if it was not of "net benefit" to Canada.
Last month, Potash Corporation asked a US District Court in Chicago to block the bid, on the basis of BHP's "false statements and half-truths".
BHP said the lawsuit was "entirely without merit".
Potash - the common name for various potassium compounds and potassium-bearing materials - is used for fertiliser production worldwide.