For the latest developments on Suncor's hostile takeover offer, see Tuesday's story: Rhetoric ramps up in Suncor's hostile $4.5B bid for Canadian Oil Sands

Suncor issued a final plea to shareholders of Canadian Oil Sands (COS) on Monday to tender their shares to its hostile takeover offer, while Canadian Oil Sands issued its own "declaration of independence," urging their investors to hold on and stay the course.

"We are urging COS shareholders to act now to protect the value of their investment by tendering their shares to our offer," said Steve Williams, Suncor's president and chief executive officer, in a statement.

"We have made a full and fair offer that provides immediate value, a safer haven as compared to COS in an extremely difficult market environment and significant upside when commodity prices finally improve," he said.

"That said, we can only invest so much time and money in this effort and will feel compelled to move on to other opportunities if we don't see substantial support for our bid on Friday," Williams warned.

Bid turned hostile in October

Suncor took its $4.3-billion all-stock offer for Canadian Oil Sands directly to the company's shareholders in October, after failing to negotiate a friendly deal with COS management.

Suncor wants Canadian Oil Sands' only asset — its 37 per cent stake in the Syncrude heavy oil development in northern Alberta. Suncor already owns 12 per cent of Syncrude.

Suncor's CEO said in his Monday note to COS investors that the prospects for Canadian Oil Sands to thrive as an independent company "have worsened considerably" since it first approached COS 10 months ago because of the current low oil price environment.

But COS management issued what it called its own "declaration of independence" on Monday as a further rebuttal to Suncor's hostile bid.

"You invested in Canadian Oil Sands for a pure-play exposure to oil prices, and you have held your investment through unprecedented hard times in the energy sector," said COS chair Don Lowry in a note to shareholders. The company also took out full-page newspaper ads to argue its case that the best option for COS shareholders is for them to refuse to tender to the Suncor offer.

"COS has the financial resources to weather the current downturn. Oil prices will recover," Lowry said. "Shareholders have more upside owning 100 per cent of COS' reserves than less than eight per cent of Suncor's."

Oilpatch riveted by fight for COS

The battle for the future of Canadian Oil Sands has at times been bitter and the oilpatch has been riveted by the clash between the two Syncrude partners.

In a Canadian Press interview, Suncor CEO Steve Williams insisted that Suncor has been playing nice.

"We've tried to keep this friendly," Williams said. "Unfortunately, Canadian Oil Sands didn't want a friendly arrangement."

Canadian Oil Sands, for its part, predicted that Suncor would engage in last-minute fear-mongering. "Make no mistake, Suncor will try to instill fear in the final hours before its bid expires," read Monday's public note from COS to its shareholders.

"But remember, Suncor will only resort to this tactic because it desperately wants what you have."

The deadline for COS shareholders to tender their stock to Suncor's offer is 6 p.m. MT on Friday. Suncor needs at least two-thirds support from COS shareholders for the bid to succeed.