Suncor Energy raised the stakes Thursday in its proposed hostile takeover of Canadian Oil Sands, saying it is willing to walk away if the Alberta Securities Commission upholds an extension to its deadline of Dec. 4 on whether to accept the deal.

Steve Reynish, Suncor’s executive vice-president of strategy and corporate development, testified that his company would not let its $4.5-billion all-stock offer stand if the regulator extends the 60-day deadline it set out when it made its bid.

The commission heard arguments into whether COS should be allowed to keep its defence against hostile takeovers, which establishes that a bid must be open for 120 days.

Suncor Energy took the offer directly to shareholders on Oct. 5 after attempts at inking a friendly deal were rebuffed by COS leadership in the spring.

In response, COS enacted a new shareholder rights plan — also known as a poison pill defence — designed to buy it more time to find alternatives to Suncor’s offer.

Suncor argues the shareholder rights plan should be struck down and that COS shareholders should have the opportunity to decide for themselves whether to take the deal.

But an affidavit filed on behalf of COS says 25 other parties have looked into possibly investing in COS, with four “highly credible” ones having signed confidentiality agreements.

COS director Arthur Korpach said Thursday the matter boils down to giving his company’s shareholders the time needed to assess all options.

“The real issue is: What is the time that’s appropriate and needed to complete the process with our third parties so that we can bring forward choice for our shareholders, as opposed to no choice?” Korpach testified.

COS has derided the Suncor offer as too low, opportunistic and exploitive.

But Suncor has warned COS shareholders that failing to accept its offer is a risky proposition, given the likelihood of a prolonged downturn in oil prices. Suncor has also described its offer as “full and fair” and has signalled it won’t be sweetening the deal.

In its filings with the Alberta regulator, Suncor called the creation of a new shareholder rights plan in the face of the offer was “an improper defensive tactic” that the regulator should not allow to remain.

Both companies are partners in the Syncrude oilsands project north of Fort McMurray, Alta. COS has a 37 per cent stake, which is its main asset. Suncor has a 12 per cent share of Syncrude and has vast oilsands operations in northeastern Alberta.

The hearing continues Friday.