United Airlines said that, unless it succeeded in cutting its costs, it would be forced to file for Chapter 11 bankruptcy protection from creditors.

The firm has given itself 30 days to conclude talks with staff on rebuilding the carrier around a new business plan.

"The changes we need to make are urgent significant and immediate," chairman and chief executive Jack Creighton said.

The carrier, facing imminent debt repayments of $875m, was already preparing for Chapter 11 in case the negotiations failed.

"Unless we lower our costs dramatically, filing for bankruptcy protection will be the only way we can ensure the company's future and the continued operation of our airline," Mr Creighton said.

Sector slump

The warning comes as US airlines continue to struggle with the after effects of the 11 September attacks, which have seen passenger numbers slump.

"The world has changed," Mr Creighton said.

"Revenue isn't coming back the way the industry expected. Demand isn't returning, fares remain low, and the industry is grappling with how to respond."

On Monday, US Airways, the sixth biggest carrier in the US, filed for bankruptcy protection.

And on Tuesday, American Airlines said it was cutting 7,000 jobs and scrapping plane orders in a cost-cutting drive.

Wage cuts

United's statement follows growing fears that the Air Transportation Stablisation Board, set up to help airlines after the 11 September strikes, was baulking at a request to guarantee $1.8bn of a $2.0bn loan.

"We've gotten the clear feeling that we need more participation from all of our stakeholders in our cost-cutting efforts," Mr Creighton said earlier.

The firm said its latest cost-saving proposals, which will be presented to staff "in the coming days", are designed to offer the "significantly broader, deeper and longer term cost savings" demanded by the ATSB.

The package follows continued attempts by United, which is 55% owned by employees, to cut wages by about 10% in return for shares and promises of future wage rises.

So far the airline has won agreement only with pilots, who were earlier on Wednesday reported to be growing impatient with unions representing other staff.

Pilots spokesman Steve Derebey said: "We just wish that the other employee groups would get on board as we have and face reality.

"It's time for the others to wake up and smell the coffee."