An Alberta regulator struck a "major blow" to rival Suncor in its hostile takeover bid by confirming shareholder rights, Canadian Oil Sands Ltd. said.

Canadian oil producer Suncor made a $3.2 billion takeover bid for Canadian Oil Sands in October. Suncor is a minority stakeholder in the Syncrude oil venture in Alberta. Canadian Oil Sands holds a 37 percent stake in the project.

In November, Suncor asked the Alberta Securities Commission to consider a shareholder rights plan adopted by Canadian Oil Sands. Suncor wanted to limit shareholder consideration to 60 days, half the time sought by Canadian Oil Sands. The Alberta regulator ruled for a 90-day window.

"This is a big win for Canadian Oil Sands' shareholders and a major blow to Suncor's credibility," Chairman Donald Lowry said in a statement. "The commission's decision applies the reins to Suncor, who tried to stampede our shareholders."

Canadian Oil Sands had said it was frustrated with the timing of the Suncor offer, noting it came during budget and spending considerations for the Syncrude project. For its part, Suncor, in a letter to rival shareholders, said a "do-nothing" stance is a risky position given the continued weakness in the energy sector.

Suncor had no comment on the decision from the Alberta regulator. Canadian Oil Sands noted the offer, as it stands, undervalues the company and it would do better as an independent entity.

"The board is continuing to aggressively examine potential alternatives such as superior offers from other parties or continuing as an independent company in order to ensure maximum value for shareholders," Lowry added.