Rio Tinto Group, the world's third-largest mining company and the owner of Kennecott Utah Copper, asked U.K. regulators to force BHP Billiton Ltd. to turn its $138 billion takeover proposal into a formal bid for the company or step away from the offer.

The U.K. Takeover Panel Executive Panel may respond by requiring BHP to make an offer or announce it doesn't intend to make a bid for the company, London-based Rio said Monday in a statement to the Australian Stock Exchange.

BHP, the world's largest mining company, proposed an offer of three of its shares for each Rio share on Nov. 8, seeking to create a company with a third of the world's iron-ore market and more energy coal, copper and aluminum than any of its rivals.

Samantha Evans, a Melbourne-based spokeswoman for BHP, declined to comment on the announcement.