Ferrovial gave the the support services group's investors until 16 May to agree to the £81m deal, but had received the support of only 77% by that deadline. It has since offered a seven-day extension.
Ferrovial issued a statement to the stock exchange that said it retained the option to grant further deadlines but that it was unlikely to do so. A spokesperson for Ferrovial said the company required approval for the deal 23 May in order to buy back Amey's £60m stake in the London Underground public–private partnership deal.
Amey, which is £190m in debt, sold the stake to consortium partners Bechtel and Jarvis at the height of its financial troubles in December, but retained the option to buy it back in six months. The deadline for taking up that option expires at the end of next month.
Ferrovial said that more delays could jeopardise completion of the deal because finalising the legalities of the takeover might take it beyond the PPP buy-back deadline.
The spokesperson said: "This is critical for us. If we extend [the approval period] by one more week we are in trouble on issues like the Underground. We are in a hurry."
Ferrovial added that it expected to obtain the support of the extra shareholders before the deadline. City analysts said the deal should be wrapped up this week. Old Mutual analyst Steven Rawlinson said: "Once the major shareholders, Meditor and Sterling, agreed to the deal Ferrovial, effectively got control. The other shareholders should fall in line."
If the sale is completed it will be the culmination of a turbulent 14 months for Amey. In March last year the firm posted an £18.3m loss, when the market had expected a £50m-plus profit. In December the company issued two profit warnings, and chief executive Brian Staples resigned in January.
A management team has been installed, led by chief executive Mel Ewell. He will stay on after the takeover.