Mining giant BHP Billiton will have to decide by 6 February whether to make a bid for rival Rio Tinto, the UK's Takeover Panel says.
Rio Tinto wants certainty for its shareholders
London-based Rio Tinto had asked the panel to force BHP to formalise its all-share bid, worth around $140bn.
Rio quickly spurned BHP's offer, saying it fundamentally undervalued Rio and its growth prospects.
BHP announced its plan in November, seeking to create a business with a third of the world's iron-ore market.
"Our shareholders deserve to have certainty and therefore we welcome the Panel's decision today," said Paul Skinner, chairman of Rio Tinto.
"The boards are unanimous in the view that BHP Billiton's rejected proposal was wholly inadequate.
"We have been very clear as to where we stand and feel it is time for BHP to do likewise."
The "put up or shut up" ruling by the Takeover Panel requires BHP, the world's largest mining company, to make a bid by 1700 GMT on 6 February or walk away.
BHP said it was considering its options.