Members of the British union voted down United's final contract offer by 97 percent in a July workplace vote, which is required in England in addition to a mail-in vote. The mailed votes will be counted Aug. 18, and if the contract is again declined the union could strike as early as one week later.
If the London workers bring picket lines to U.S. airports, members of the International Association of Machinists and Aerospace Workers will not cross them, said Randy Canale, president of the IAM district for United.
Officials at United's pilots and flight attendants unions said they would support the British workers as well. Their contracts say that members are not required to cross other employees' picket lines.
The IAM went a step further, saying that if United finds people to handle striking workers' jobs in moving flights out of London's Heathrow Airport, IAM workers would not take care of the flights on the U.S. side, Canale said.
The potential for a strike in England could threaten some of United's most profitable routes. The airline's international flights have consistently boosted its revenues during its 20-month bankruptcy. United flies 12 round trips between Heathrow and U.S. destinations each day, including three to O'Hare.
United spokesman Stephan Roth said Tuesday that the airline has "made a fair offer to our employees, and we hope they'll accept this offer."
The possible strike represents another labor challenge for the nation's second-largest airline.
Its U.S. unions have spoken out angrily against United's decision in July to stop funding its pension plans while it remains under Chapter 11 protection. Many believe the airline intends to terminate the plans, which the government estimates are underfunded by at least $7.5 billion.
Canale, also a member of United's board of directors, boycotted its last two meetings because the airline was discussing its decision to stop making pension payments. He plans to attend a board meeting Thursday.
The IAM also has filed two lawsuits against top United executives, accusing them of neglecting their fiduciary duties to employees and creditors by stopping pension contributions.
United says it needs the extra cash while it searches for exit financing in the wake of the government's rejection of its application for a $1.1 billion federal loan guarantee that the airline had counted on to propel it out of bankruptcy.