The board of Canadian Oil Sands Ltd. is telling shareholders to reject Suncor Energy’s $4.3-billion takeover offer, saying it exploits inside information about an oilsands mine partly owned by both companies.

Board members have unanimously decided the hostile bid, made two weeks ago, is not in shareholders’ best interests, COS said Monday.

COS CEO Ryan Kubik said Suncor is looking to capitalize on improvements that are in the works at Syncrude — a sprawling oilsands development north of Fort McMurray, Alta. — before they are reflected in his company’s share price.

“They understand the improvements already underway and want to take credit for them,” he told a conference call, which did not include a question-and-answer session.

COS is the largest owner of Syncrude with a 37% stake — its main asset. Suncor, Canada’s dominant oilsands player, has a 12% interest.

The COS (TSX:COS) board is also accusing Suncor of taking advantage of “unprecedented” turmoil — not just because of the crude downturn, but also because of political and regulatory uncertainty.

Kubik said Suncor is asking shareholders to “give them your upgrader for free,” by failing to factoring in Syncrude’s ability to process oilsands bitumen into more valuable and easier-to-refine synthetic crude.

As well, Kubik said the Suncor bid does not account for an eventual recovery in oil prices, which are less than half what they were in mid-2014

Both Kubik and COS chairman Donald Lowry took aim at recent comments by Suncor CEO Steve Williams, who said his company would be considering acquisitions at “fire sale” prices amid the crude slump.

“Your board’s objective is to maximize value for shareholders, not to sell you out at fire sale prices,” said Lowry.

Williams said none of COS’s reasons for rejecting the bid detracts from its “compelling” value.

“Our offer reflects the new business reality and when proposed, included a substantial price premium of 43% and a dividend increase of 45%,” Williams said Monday in an emailed statement.

“It also represents an opportunity for investment in a financially stronger, more diversified and stable company that has considerable upside potential in a rising price environment, but can also deliver significant value should oil prices stay lower for longer.”

Suncor (TSX:SU) approached COS in the spring with an offer that was at the time worth $11.84 a share. Kubik described that offer as “barely credible” and a “takeunder” rather than a takeover.

The current all-stock bid, which has been taken directly to COS shareholders, was worth $8.84 a share when it was made on Oct. 5.

In a research note, Desjardins Capital Markets analyst Justin Bouchard said the current offer is too low, but it’s “in the ballpark.”

A 16% increase in its value would be fair, he said.

“We believe the offer is opportunistic (as it should be!), but note that Suncor is the only logical buyer.”

COS has enacted a new shareholder rights plan — also known as a poison pill defence — to dissuade Suncor from buying up its target’s shares.

The COS board and its advisers are continuing to look at other options, ranging from remaining an independent company to a deal with another firm.