Transatlantic routes face possible disruption as workers employed by United Airlines at Heathrow vote on strike action in a row over pay.
The Unite union said that a real terms pay cut and outsourcing threats had provoked a ballot for industrial action.
The ballot has opened and will run for three weeks until October 11.
If the workers back industrial action it would mean the cancellation of up to 18 flights a day from Heathrow, according to the union.
Unite represents almost 300 workers in operations, customer services and baggage handling at United Airlines.
It comes as engineers for American Airlines at Heathrow also vote for a strike in a ballot that closes on September 30.
Unite claimed that United Airlines workers based at Heathrow are being offered a pay increase worth just 5% plus an additional payment this year, and 4% in 2023 at a rime when inflation (RPI) is currently at a 40 year high of 12.3%.
The workforce has also been angered by the carrier threatening to tear-up a long standing agreement which bars the use of outsourced labour or third party companies.
Unite general secretary Sharon Graham said: “United Airlines is offering workers a pay cut when it can afford to pay its staff fairly.
“To make matters worse, bosses want to tear up a long standing agreement barring the use of outsourced workers.
“These are red lines the workers won’t cross so the airline faces the prospect of steadfast industrial action. Unite members have their union’s complete backing to protect their pay, terms and conditions.”
Unite regional officer Clare Keogh added: “Unite has a long standing commitment from United Airlines to prioritise direct employment. Now management want to open the floodgates to agency and outsourced labour. Our members want fair pay and they won’t allow a race to the bottom.
“The workforce feel they have no other choice except to begin an industrial action ballot. It’s time for United Airlines to think again.”
A spokesperson for the airline said: “United Airlines continues to hold talks with Unite the Union, with a view to reaching an agreement.”