NEW YORK (CNN/Money) - Shares of United Airlines' parent UAL Corp. tumbled 33 percent Friday after the carrier's mechanics rejected their part of a $1.5 billion package of wage concessions, a move that some fear could force the airline into a bankruptcy filing and prevent it from qualifying for an almost $2 billion federal loan package.

In addition, ratings agency Standard & Poor's downgraded its long-term corporate credit rating on UAL to CCC- from CCC, saying bankruptcy is virtually inevitable.

The company had sought the vote on the concessions as part of an overall cost-cutting plan needed to secure $1.8 billion in federal loan guarantees.

UAL (UAL: Research, Estimates) stock tumbled $1.18, or 33 percent, to $2.45 Friday.

United's 13,000 Mechanic and Related employees, who are members of the International Association of Machinists and Aerospace Workers (IAM), rejected the package, with 57 percent voting against it, even though the union had recommended its acceptance.

When contacted by CNNfn, Joe Tiberi, Machinists spokesman, would not say why mechanics had rejected the proposal.

United President and CEO Glenn Tilton said he was disappointed by the mechanics' decision but would work to bring another contract proposal before the group quickly.

"Clearly, we're disappointed that our mechanic and related employees, represented by IAM District 141-M, did not approve the tentative agreement with United," Tilton said. "Nevertheless, we remain fully committed to the goals of the United Airlines Union Coalition in achieving labor cost savings that will enable us to secure federally backed loans."

High-level talks are taking place Friday between UAL and the machinists union, UAL spokesman Jeff Green told CNNfn.

"I wouldn't be surprised if it passed. Maybe the machinists are just trying to send a message to everybody," Ray Neidl, an airline analyst at Blaylock & Partners, told CNNfn. "Also, the machinists are a big part of the skilled workers and maybe some of them thought they could do just as well if they left United -- if United were to go out of business."

Two other groups of United employees who are IAM members and who represent 24,500 workers -- including customer-service agents, baggage handlers and reservation agents -- approved their portion of the wage-concession package. Ramp & Stores, Food Service and Security Guards ratified their agreements with 63.4 percent of voters approving the accords. The Public Contact Employee (PCE) group, meanwhile, also accepted their agreement, with 79.2 percent endorsing the contract, IAM said.

"Each employee measured the costs and benefits of participating in United's recovery plan," said Scotty Ford, IAM District 141-M president, in the association's statement. "In the end, some thought the risk was worth taking, and others felt they had sacrificed enough. We respect both decisions and this organization will aggressively represent their common interests as this extraordinary situation unfolds."

Saturday brings a vote by the rank and file of the Association of Flight Attendants. That labor deal would save the airline about $400 million. The pilots have already ratified a labor deal designed to save the airline $2.2 billion.

United recently secured $5.2 billion in wage cuts from its employees, including five separate unions, as part of a financial recovery plan put before the Air Transportation Stabilization Board (ATSB). The federal agency was created after the Sept. 11 attacks and charged with doling out up to $10 billion in loan guarantees.

Elk Grove Village, Ill.-based United, a unit of UAL Corp. (UAL: down $1.18 to $2.45, Research, Estimates), has asked the agency to back $1.8 billion of a $2 billion loan. It has met with staffers every week after the board said more labor wage concessions were needed than what the airline originally outlined.

Industry experts say a decision from the ATSB will determine the near-term fate of United as it tries to avoid restructuring through the courts.

The airline has $375 million in bond payments due Monday, and it is not expected to make that payment without some positive sign on the federal loan guarantee. The company could buy time on the loan by invoking a 10-day grace period before a default, making the new deadline Dec. 16.

United has been burning through more than $7 million cash a day, said Philip Baggaley, the Standard & Poor's analyst who downgraded the company's rating. By doing so, it is quickly eating through the $1.7 billion it had in reserve at the start of the fourth quarter.

Baggaley told CNNfn said that he believes there is a 90 percent chance UAL will have to file for bankruptcy.  

--Reuters contributed to this report