Rio Tinto has asked the Takover Panel to force BHP Billiton, its rival mining giant, to make a formal offer to buy it or withdraw.

The Anglo-Australian miner has requested a “put up or shut up” ruling that would put an end to weeks of speculation about Rio's future that has dogged the company since BHP revealed early last month that it had proposed that the two companies merge.

Rio rejected the share-for-share proposal that today values it at about £67 billion and Tom Albanese, Rio’s chief executive, said on Friday, that BHP’s bid was “dead in the water” because it failed to reflect Rio’s value. Marius Kloppers, BHP’s chief executive, has engaged targeted investors and customers of Rio in a marathon charm offensive, seeking support for his merger proposal.

The BHP proposal has provoked a storm of protest from Asian steelmakers, which depend heavily on the miners’ operations in Australia for supplies of iron ore. Meanwhile, rumours have surfaced of other bidders that might intervene in BHP’s attempt to woo Rio. Last week, Xu Lejiang, the chief executive of Baosteel, appeared to confirm in Chinese news reports that China’s largest steelmaker was considering an offer for Rio.

Blackstone, the private equity group, has vigorously denied reports that the company was about to mount a bid for Rio. Blackstone said that it had “consulted with the UK Takeover Panel and told them that there is no truth to this story”.

The Takeover Panel said yesterday that its policy was not to comment on its discussions with companies in bid situations. Under the Panel’s Rule 2.4(b), it can at the request of an offeree company order a potential offerer to make a formal bid within a specified time or indicate that it is no longer interested in bidding.

Typically, the offerer would be given between six and eight weeks to make its bid. A decision to withdraw would be expected to bind the potential offeror for a period of at least six months, but companies are able to attach conditions to the withdrawal, relating to changed circumstances or a subsequent agreement with the board of the target company.

Rio’s iron ore operation in the Pilbara region of Australia is the key to a BHP merger proposal, being the company’s most valuable asset and the most controversial part of the merger equation.

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