Seymour Schulich said shareholders should not sell at the current price offered and that he believed Suncor was willing to pay more. Schulich, who could not immediately be reached for comment, reportedly owns about 5 percent of Canadian Oil Sands, which would make him one of its largest investors.

"Suncor is trying to pull a fast one on all of us," Schulich wrote. "Quite simply, they're offering an unacceptable price for an irreplaceable asset."

Last week, the board of Canadian Oil Sands sent a letter to shareholders urging them to reject Suncor's offer because it undervalues the company.

With a 36.7 stake in Syncrude, Canadian Oil Sands is the largest-interest owner in the joint venture Syncrude mining and upgrading project, which has capacity to produce up to 350,000 barrels per day, making it the largest single source of crude production in Canada.

However, the project has been dogged by operating issues. Suncor says that by taking a larger stake in the project it would be able to improve production rates.

Suncor currently owns 12 percent of Syncrude, a stake that would rise to 49 percent if its bid were successful.

In his letter, Schulich argued Syncrude had reduced costs in a low oil price environment and shares in Canadian Oil Sands were well positioned to slingshot higher when the price of oil rebounds. Global crude prices have plunged around 70 percent in 18 months, with U.S. crude last trading at $36 a barrel.

Suncor Chief Executive Steve Williams said he respected Schulich's perspective but did not think it reflected of the broader view of shareholders.

"The indications this week is that the majority of shareholders are going to support," Williams said on a conference call, in which he once again urged investors to tender to the bid.

"The choice is yours, you can accept Suncor's offer and capture a significant premium while retaining oil sands exposure ... or you can absorb an immediate share price decline, possibly of 40 percent or more, and hope for an oil price recovery."

Canadian Oil Sands shares were last up 19 cents on the Toronto Stock Exchange at C$8.08, more than 30 percent from their closing price before Suncor's offer on Oct. 5. However, the share price has slid over the last week as Friday's bid deadline nears.

Suncor was last trading down 35 cents at C$34.93.

(Reporting by Nia Williams; Editing by Steve Orlofsky)

By Nia Williams

Stocks treated in this article : Canadian Oil Sands Ltd, Suncor Energy Inc.