CALGARY, Alberta (Reuters) - Canadian Oil Sands (>> Canadian Oil Sands Ltd) shareholders will have an extra month to consider a hostile takeover bid from Suncor Energy Inc (>> Suncor Energy Inc.) but a plan to extend the deadline to February was rejected on Monday.

The Alberta Securities Commission said shareholders should have until Jan.4, 2016 to decide how to respond to the offer. However, it struck down a plan adopted by Canadian Oil Sands in October that extended the period for bids to be considered to 120 days from the original 60-day time frame.

"The commission met everyone halfway," said Bradley Freelan, a partner with Fasken Martineau and an expert on hostile bids.

Canadian Oil Sands adopted the new shareholder rights plan, also known as a poison pill, two days after Suncor made its unsolicited all-stock offer in early October.

The plan was seen as a stalling tactic to enable other bids to be assessed in the hope that a "white knight" bidder emerged.

Since then, Canadian Oil Sands has said around 25 other parties have shown interest, although no other offers have yet emerged.

ASC vice-chair Stephen Murison said the panel had been told other potential bidders needed longer to work on the process of pulling together rival offers.

"We accept that additional time after the end of this week will be useful and perhaps necessary for the process to do its work in the interest of COS shareholders," Murison said.

Suncor's C$4.3 billion bid for Canadian Oil Sands is due to expire on Friday and spokeswoman Sneh Seetal said Suncor was reviewing the decision to determine its next steps.

Canadian Oil Sands said the ASC's decision was a major win for its shareholders and reiterated that it thought Suncor's offer was substantially undervalued.

"The ASC decision applies the reins to Suncor, who tried to stampede our shareholders," said Donald Lowry, chairman of Canadian Oil Sands. "This decision exposes Suncor's opportunism and levels the playing field."

Canadian Oil Sands is the largest-interest owner in the Syncrude oil sands mining and upgrading project, which has averaged 242,000 barrels per day of crude so far this year, although total capacity is around 350,000 bpd.

Suncor currently owns 12 percent of the joint venture project. The other partners are Imperial Oil (>> Imperial Oil Limited), Mocal Energy, Murphy Oil (>> Murphy Oil Corporation), CNOOC (>> CNOOC Ltd) subsidiary Nexen and Sinopec <0386.HK>.

(Additional reporting by Matt Scuffham and John Tilak; Editing by Tom Brown and Andrew Hay)

By Nia Williams