CALGARY -- Canadian Oil Sands Ltd. is looking to buy time as it faces a hostile takeover offer from oilsands giant Suncor Energy Inc., announcing Wednesday that it's adopting what's known as a poison pill defence.

The new shareholder rights plan could be triggered if anyone buys 20 per cent or more of COS (TSX:COS) outstanding shares. At that point, other shareholders in the target company can buy stock at a discount, making the acquisition less attractive to a hostile bidder.

Bids permitted under the new shareholder rights plan must be open for 120 days. Since Suncor $4.3-billion all-stock offer is open until Dec. 4, COS said it doesn't pass muster.

Suncor (TSX:SU) announced on Monday it would be taking its offer directly to COS shareholders following two rebuffed attempts. It had approached COS in March and April in the hopes of inking a friendly deal.

Suncor's offer would also see it taking on $2.3 billion in COS debt, bringing the total pricetag to $6.6 billion.

The current offer is worth substantially less than the earlier overtures -- $8.84 a share as of Friday's close versus $11.84 on March 31.

The outlook for crude prices has deteriorated since Suncor's earlier attempts. Although West Texas Intermediate crude has nudged closer to the US$50 a barrel mark in recent days, Suncor sees a "lower for longer" scenario.

"The board will consider Suncor's unsolicited offer in both the current context and in light of the strong long-term potential of Canadian Oil Sands," said COS board chairman Donald Lowry.

"Shareholders do not need to take any action or make any decision about the Suncor offer until the board has had an opportunity to fully review the offer and to provide a recommendation based on careful analysis."

COS is the largest partner of the massive Syncrude oilsands mine in northeastern Alberta, with a 37 per cent stake. Suncor has a 12 per cent interest in the development.

If Suncor -- already the dominant oilsands producer -- is successful in taking over COS, it would own just under half of Syncrude.

Suncor and Syncrude are the oldest operators in the oilsands, with side-by-side mines north of Fort McMurray, Alta.

Some observers have said ExxonMobil-controlled Imperial Oil (TSX:IMO), which has a 25 per cent stake in Syncrude and manages its day-to-day operations, may look to top Suncor's offer.