SYDNEY, March 23 --  An $8.7 billion bid for Qantas Airways Ltd. , the world's biggest airline takeover, was in disarray on Friday after a key shareholder said it would vote against the offer, forcing the buyout group to rethink its deal.

Shares in the Australian carrier slumped as much as 6 percent on fears the deal would collapse after fund manager Balanced Equity Management said it did not plan to accept the offer from a consortium led by Macquarie Bank .

While Balanced Equity holds about 4 percent of Qantas, analysts said there were now fears other shareholders would also reject the bid which needs the backing of 90 percent of shareholders to succeed.

The bid consortium, Airline Partners Australia, said in a statement it was considering a "range of alternatives" in response to Balanced Equity's decision. It cannot legally raise its offer.

The sale of the national icon, dubbed the flying kangaroo, has been backed by the Australian government despite some political and union opposition.

"About 60 percent of the stock is now held by hedge funds and other institutions and it's difficult to know what their intentions are," said Derek Sadubin, an analyst at the Centre for Asia Pacific Aviation.

"This (rejection) would suggest that the momentum behind the offer is unwinding."

The bid consortium is raising $8.4 billion in debt to fund the buyout, and the 90-percent requirement is an important safeguard for debt holders.

Qantas shares fell 6.3 percent, but recouped some of the losses to close down 3.1 percent at A$5.06, still some 7 percent below the A$5.45-a-share bid price.

The fate of the bid rests with UBS Global Asset Management which has privately criticised the offer as too low, according to local media reports.

The Australian Financial Review said on Friday that UBS owned about 6 percent of Qantas but had placed orders for more stock. A UBS spokeswoman declined to comment.

UBS's investment banking division is advising Qantas on the deal.

"I think it's time UBS told us what they're going to do," Argo Investments Fund Manager Rob Patterson Patterson said.

"They've been saying the offer is not high enough but they haven't said they won't accept it."

The bidding consortium -- which also comprises private equity firm Texas Pacific Group [TPG.UL], Allco Equity Partners , Allco Finance Group and Canadian investment firm Onex Corp. -- said about 29 percent of Qantas stock had so far accepted its bid, and extended the offer until April 20.

It declined to comment on the alternatives it would consider. Analysts said it could reduce the 90 percent limit to 50 percent but would then face challenges restructuring the funding.

There have been growing signs of shareholder resistance to private equity buyouts in Australia. Shareholders rejected a A$1.6 billion bid for travel retailer Flight Centre last month.

Qantas has issued three profit upgrades in the last six months as fuel prices ease and demand for travel increases.