LONDON, May 15 (Reuters) - Mexico's Fresnillo Plc (FRES.L), the world's largest primary silver producer, has asked a Canadian authorities to set a three-week deadline for the completion of a valuation of MAG Silver (MAG.TO) so Fresnillo can make a formal takeover bid.

London-listed Fresnillo announced on Dec. 1 its intention to make a cash bid worth $179 million for the 80.2 percent of Toronto-listed MAG it does not already own. [ID:nBNG120765]

Fresnillo, which had previously said MAG Silver was using delaying tactics, said in a statement on Friday it had asked the Ontario Securities Commission (OSC) to set a three-week time frame for MAG Silver to complete a long-delayed independent valuation.

"If a valuation is not forthcoming in this time frame, Fresnillo has asked for an exemption from the valuation requirement, so as to enable (its unit) Fresbal to make its offer directly to MAG shareholders without further delay," the statement added.

MAG Silver has argued that the valuation has not be completed because it was waiting for key information that Fresnillo has not provided.

On May 8, MAG Silver said it had asked the OSC to force Fresnillo to provide the requested information and also said it had made a formal request for arbitration with the International Court of Arbitration. (Reporting by Eric Onstad; editing by Simon Jessop)