The Unite union is balloting United Airlines members at Heathrow on whether they want to strike over pay and concerns around outsourcing.
The ballot is open and will run for three weeks until 11 October. If successful, Unite could – at the very earliest – call a strike starting 25 October.
Half-term runs form Monday 24 October to Friday 28 October.
Unite represents almost 300 United workers in operations, customer services and baggage handling at Heathrow. It is warning that if members back industrial action, it could impact up to 18 flights a day from Heathrow.
Unite is also balloting American Airlines members at Heathrow on strike action; a successful vote could impact more than 20 daily AA departures.
The union said despite United reporting quarterly profit of US $329 million, the airline is offering its Heathrow workers a pay increase worth 5% and 4% next year against a backdrop of 10%+ inflation.
Additionally, Unite said members were concerned about the longevity of a long-standing pledge by United not to use outsourced or third-party labour.
"This is a red line for Unite members," said the union. "If the airline pressed ahead, it would undo years of mutual agreement."
Unite general secretary Sharon Graham said: “United Airlines is offering workers a pay cut when it can afford to pay its staff fairly. Unite members have their union’s complete backing to protect their pay, terms and conditions."
