BHP Billiton Ltd says it is still seeking to engage in discussions with Rio Tinto Ltd after regulators gave it 47 days to either launch a formal bid for its rival or walk away.
The February 6 deadline (February 7 AEDT) was set early Saturday morning by the UK Takeover Panel after Rio Tinto successfully invoked Britain's so-called "put up or shut up" laws.
The laws allow a company subject to a potential takeover bid to request that a time limit be imposed on potential buyers to clarify their intentions.
"BHP Billiton is considering its options in light of the deadline set by the panel executive but no decision has yet been taken on this matter," BHP said in a statement.
"Accordingly, there can be no assurance that BHP Billiton will progress its proposal to Rio Tinto or that any offer for Rio Tinto will be made.
"BHP Billiton continues to seek to engage in discussions with Rio Tinto with a view to obtaining the support and recommendation of the board of Rio Tinto for its proposal."
BHP has already made an informal offer to Rio Tinto of three BHP shares for every one Rio Tinto share, valuing the target at around $150 billion.
But the proposal was swiftly rejected by Rio Tinto's board, which said it "significantly" undervalued the company.
Most analysts believe that BHP will have to produce a much higher offer to succeed.
Rio Tinto welcomed the Takeover Panel's decision.
"We have been very clear as to where we stand and feel it is time for BHP to do likewise," Rio Tinto chairman Paul Skinner said in a statement.
"Our shareholders deserve to have certainty and therefore we welcome the panel's decision."
Rio Tinto is the world's third biggest miner. A merger with BHP would create the world's largest producer of coking coal, thermal coal, copper and aluminium.
The $400 billion combined group would also have similar iron ore output to leading global iron ore producer CVRD.
Although a bid from BHP can be expected any time now, CommSec chief equities economist Craig James said current turbulence in the global debt market could put it off until later.
"BHP have to talk to a number of parties if they've got to take on debt," Mr James said.
"But it could be the case that BHP walks away for now and comes back later in the year. There's no rush.
"But it makes a whole lot of sense for BHP and Rio to get together because it's hard to develop new mines. It's hard to get the personnel. It's hard to get the equipment."