A joint-venture partner is suing BHP Billiton over its plan to sell its stake in the Ekati diamond mine and surrounding exploration properties to Harry Winston Diamond Corp.

C. Fipke Holdings Ltd. (Fipco) has accused BHP Billiton of not complying with its pre-emptive rights to buy the assets under the joint-venture agreement and alleged Harry Winston was complicit.

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The Ekati Spirit, a 78-carat diamond mined from BHP Billiton's Ekati mine in the Northwest Territories, was auctioned for more than $6 million in Antwerp, Belgium, in 2011. (BHP Billiton)

Fipco also alleges Harry Winston's financing arrangements for the acquisition have interfered with Fipco's ability to arrange its own financing. It is seeking a court order blocking the sale unless and until BHP Billiton provides Fipco with revised offers.

Harry Winston said the company and BHP Billiton believe the claim is "entirely without merit, and intend to vigorously defend the action in order to proceed with the purchase and sale of the Ekati Mine."

The company said the various groups have agreed to have the case heard on an expedited basis with a goal of a final ruling no later than April 15.

The Ekati diamond mine is located about 300 kilometres northeast of Yellowknife in the Northwest Territories.