The UK Takeover Panel has given a consortium led by Australia's Macquarie Bank until 15 December to make a bid for the London Stock Exchange (LSE).
Macquarie said in August it was looking for partners to launch a possible takeover bid for the LSE.
In a statement the Sydney-based bank says it has continued to review the merits of a formal approach since then, but the increase in speculation and its impact on LSE's share price has made any assessment of the fundamental value of the UK stock exchange "increasingly challenging".
Shares in the LSE have risen more than 70% since September last year on the back of growing speculation about the Exchange's future. Market analysts have implied that it is the premium on the LSE's shares that has so far hampered any deal.
Macquarie says it will only invest in businesses where value exists for its investors and other stakeholders and there can be no assurance that any offer will be made.
The bank now has until 5.00pm on Thursday 15 December to declare its intentions. If Macquarie decides not to bid it will be barred from making any further approaches to the LSE for six months.
However, the put up or shut up deadline has faded market hopes, and LSE shares fell one per cent to 598 pence on the news.
Earlier this month Britain's Competition Commission (CC) gave the go ahead to Germany's Deutsche Börse and pan-European exchange Euronext to bid for the LSE on condition that they ensure the UK exchange's clearing provider remains independent.
Both exchanges launched bids for the LSE late last year but Deutsche Börse withdrew its £1.35bn offer after it was rejected by the LSE and following a shareholder revolt which eventually led to its chief executive being ousted in May. Euronext has yet to declare the price it is willing to pay for the UK stock market operator.