NEW YORK (CNN/Money) - Shares of American Airlines parent AMR Corp. plunged Monday amid new bankruptcy fears brought on by union outcry over executive compensation plans at the troubled airline.

Shares of AMR (AMR: down $0.42 to $3.43, Research, Estimates) sank 23 percent after the Association of Professional Flight Attendants vowed to take a new vote on the concession contract the company says it needs to stay out of bankruptcy court. After the stock market closed, the Transport Workers Union representing mechanics and ground workers at American said it also wanted a new vote on a concession pact it ratified last week.

American Airlines, the world's largest airline, faces a new bankruptcy threat due to union outcry over its executive compensation plans.

The agreement with the flight attendants, only narrowly approved in a vote last Wednesday, would give the company $340 million in annual cost savings over the next six years. It was part of $1.8 billion in annual labor cost savings either agreed to by the unions or imposed on nonunion employees by AMR last week.

But union leadership said late Friday that compensation plans for American's best paid executives were "morally bankrupt" and demanded another vote of members.

"This taints the agreement that was ratified just two days ago in a wrenching process for our members," said a letter from APFA President John Ward late Friday night. "Every APFA member -- those who voted for the agreement and those who voted against it -- are outraged by this action, as am I."

American Airlines spokesmen would not take questions on the APFA threat to hold a new vote.

"American has a valid ratified agreement with the APFA," was the only statement American spokesman Gus Whitcomb made.

AMR Corp. CEO Don Carty

APFA spokesman George Price said the union met with its legal advisers throughout the weekend to discuss the new vote, and that it is committed to going to membership once again.

"We wouldn't expect the company to say anything different from what they did on Friday," said Price when asked about the company's claim of a valid concession contract with APFA. "Our goal is to continue to work with the company to keep them out of bankruptcy. With all of that said, we felt reballoting is the only fair way to go."

He wouldn't comment on the chances for passage of the labor pact, which received only 53 percent approval in the vote completed Wednesday. Details of a new vote are not going to be available until Tuesday at the earliest, said Price.

But one official with another union at American, speaking on a not-for-attribution basis, said that the APFA agreement was already signed, and the union would not be able to go back on the agreement.

"It's questionable if they did a revote if it'll be legal or not. They voted, they signed. That's the facts," said the union official. An APFA spokeswoman said she couldn't comment yet on whether the union had signed the agreement with American.

The official with the other union said that the other two unions at American -- the Allied Pilots Association and the Transport Workers Union, which represents most ground workers -- have yet to sign their contracts. But he said despite anger by all the unions, he doesn't believe any of the concession pacts will be put to new votes by the unions.

"We're in the same situation that we were last week. If one of the unions doesn't sign, then we're in bankruptcy," said the union official.

If there is a bankruptcy filing, it will not have a significant immediate effect on American Airlines customers. The carrier plans to file under Chapter 11 of the bankruptcy code, which allows for continued operations. But Fort Worth, Texas-based American eventually could make deeper cuts in its flight schedule and route network to cut costs and satisfy creditors.

The employees also likely would feel deeper cuts after a bankruptcy filing. American has said that in order to win approval from lenders for $1.5 billion in loans needed to operate during bankruptcy, so-called debtor-in-possession financing, that it will need an additional $500 million in annual labor cost savings on top of $1.8 billion in cuts included in the concession contracts.

Even if the unions or their memberships reject the call for deeper cuts, the airline has the ability to ask the bankruptcy judge to impose the deeper cuts.

If a filing occurs, it likely would wipe out American's investors. Most bankruptcy filings leave little for holders of the company's equity, although shares of some companies in Chapter 11 continue to trade for weeks or even months after filing.

The airline revealed in a Securities and Exchange Commission filing Tuesday that was reported in the Wall Street Journal Thursday that its top six executives are in line for retention bonuses equal to twice their base salary if they stay with American through January 2005. A seventh officer was due 1.5 times his base pay.

It also revealed that a supplemental pension program for its top paid executives had been funded for the first time in order to protect those benefits in case of a bankruptcy filing.

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After initially defending both plans, late Friday afternoon Carty announced that he and the other top officers had given up their retention bonuses.

"I have apologized to our union leaders for this and for the concern it has caused our employees," Carty said. "Those executives who have made the personal commitment to remain with American during this financial crisis, myself included, are not here solely for monetary reasons, and we have all agreed to give up these retention payments in order to give our employees confidence in management's on-going commitment to shared sacrifice."

But Carty said the funding of the supplemental pension plan for these officers and other top paid executives would remain in place despite the union objections.

APFA and other union officials said Friday that American had done damage to the trust of employees with its attempt to set up the compensation plans.

"No doubt, this cancellation is only a result of the fact that they were caught with their hands in the cookie jar," said a statement from APFA President Ward to his members late Friday.