It is feared the airline could be forced into bankruptcy as a consequence.

Thousands of mechanics, baggage handlers and other workers were asked to accept a pay cut of between 6% and 7% in an effort to save $1.5bn in wages.

And although thousands of workers agreed to the proposal, a significant minority refused to do so.

No loan

A much needed $1.8bn federal loan guarantee may now be in jeopardy.

United Airlines would need to cut labour costs by $5.2bn over five years to get the loan from the Air Transportation Stabilisation Board, the body set up after last year's 11 September attacks.

Each employee measured the costs and benefits of participating in United's recovery plan

Scotty Ford President District 141-M

Earlier, pilots and flight attendants had agreed to pay cuts in order to stave off bankruptcy.

And union members have warned that without further cuts, the airline will not meet debt repayments.

Divided workforce

The mechanics who rejected the deal were represented by District 141-M of the International Association of Machinists [IAM].

A majority of the district's 13,000 members voted against the proposed cuts.

Their rejection rendered ineffective the acceptance by 24,000 other IAM members, including public sector employees and baggage handlers.

"Each employee measured the costs and benefits of participating in United's recovery plan," said District 141-M president Scotty Ford.

"In the end, some thought the risk was worth taking, and others felt they had sacrificed enough."

Hit by terror

United Airlines is the second largest airliner in the US.

But it has suffered badly from the sharp knock to the industry in the wake of 11 September and the rising popularity of budget airlines.

Reports suggest the airline has been losing $7m a day in recent months, and it must somehow find funds to pay off $375m in debt by 16 December.