One of two groups seeking to buy airports operator BAA has been told to reveal its intentions after having its initial interest rebuffed.
The Takeover Panel has told the Goldman Sachs-led consortium to confirm it will make a bid by 9 June or to withdraw.
BAA, whose assets include Heathrow and Gatwick airports, rejected a preliminary approach last month from the group, valued at 870 pence a share.
BAA has also rejected two firm bids from a group led by Spain's Ferrovial.
If the Goldman Sachs-led group decides not to make a firm bid it will be barred from doing so for a further six months under stock market rules.
"The panel has ruled that the consortium's statement must be clarified," the Takeover Panel said in a statement.
It added that both the consortium and BAA had agreed to the ruling.
BAA has said would-be bidders will have to pay a substantial premium above its current market value to secure control of the company because of its unique portfolio of assets.
Meanwhile, reports suggested that Ferrovial had snapped up a 15% stake in BAA, a move that would improve its bargaining strength.
Dealers told Reuters news agency that the Spanish firm's adviser, Citigroup, had bought around 15 million shares in BAA at 900p per share.
Last week, BAA rejected an improved 900p a share bid from Ferrovial saying the true value of the airport operator value was more than 940 pence per share.
By owning Heathrow, Gatwick and Stansted airports, BAA controls more than 90% of passenger traffic in and out of London.
The reports pushed BAA shares to record highs of 907p, before they settled at 903p - the first time its stocks have pushed above Ferrovial's offer price.