Immigration queues at Heathrow Airport could last up to 12 hours on 30 November as a result of strikes from Border Agency staff.

Chief operating officer for Europe’s busiest air hub, Normand Boivin, sent a letter on Friday to carriers operating at Heathrow asking them to slash loads in half to avoid “gridlock” at immigration checkpoints. Gatwick has also contacted airlines and requested that passengers be allowed to change flights.

Public service unions that represent UK Border Agency employees, health workers, civil servants and teachers are planning the strikes in response to new government measures in which the retirement age will be raised for public employees. The move will also require affected employees to contribute a greater portion to pension schemes.

According to Boivin, the immigration body has been unable to put contingency plans in place. The letter says that middle managers were called upon to fill Wednesday’s void, but they were unwilling to provide assistance after immigration chief Brodie Clark had recently stepped down amid a dispute over passport check protocol with Theresa May, the home secretary.

British Airways has since announced that passengers with bookings for Wednesday will be able to change their itineraries free of charge.