United Airlines Holdings Inc. faces potential disruption in one of its most lucrative overseas markets as 300 workers at London’s Heathrow airport begin voting in a strike ballot over pay.

The poll of staff employed in operations, customer services and baggage handling will run until Oct. 11, the Unite union said in a statement Wednesday.

United’s UK employees are in dispute with the US company over what Unite called real-terms pay cuts amid a cost-of-living crisis. It said the carrier has offered a raise of 5% this year plus an additional payment, followed by 4% in 2023, at a time when inflation is running well ahead of those figures.

“To make matters worse, bosses want to tear up a long-standing agreement barring the use of outsourced workers,” Unite General Secretary Sharon Graham said. “These are red lines the workers won’t cross.”

The union said disruption at United would threaten as many as 18 flights a day into Heathrow, the leading destination for US carriers in a trans-Atlantic market that’s rebounded faster from the Covid crisis than most other long-haul sectors.

A UK spokeswoman for United didn’t immediately comment. 

The ballot at United comes after Unite said earlier this month it would poll more than 50 Heathrow-based maintenance staff at American Airlines Group Inc. on possible strikes over a pay offer it called “appalling.” That consultation closes on Sept. 30.

In another UK labor dispute, Arriva bus drivers in north London will begin action over pay from Oct. 4, with the strike continuing until a resolution is reached, Unite said. Drivers are seeking a 12.3% hike, which the union says is in line with inflation.

Also Wednesday, the Transport Salaried Staffs’ Association said it will ballot hundreds of members at London commuter railway Govia Thameslink on possible walkouts.