An unspecified number of Border Force staff who are members of the Public and Commercial Services union will walk out for eight days from 23 December until about 7am on 26 December and again on 28 December until around 7am on 31 December.

The action will affect Birmingham, Cardiff, Gatwick, Glasgow, Heathrow and Manchester airports, plus Newhaven ferry terminal, and is likely to impact an estimated 10,000 flights, with Army and civil service personnel covering immigration posts after receiving training needed to check passports.

Steve Dann, Border Force chief operating officer, said: “Border Force has, for a number of months, undertaken extensive planning and we have been working with the travel industry and ports.

“We have a reasonable expectation that we will be able to deliver a service that keeps most, if not all, ports open,” he said, but admitted: “Our contingency workforce will not be able to operate with the same efficiency. We apologise for any disruption caused to travellers entering the UK.”

He added security would not be compromised during the dispute.

One focus for officials has been Heathrow, which is expecting 10,000 passengers to arrive before 7am on Friday. However, Border Force is known to have built this into its contingency plans.

Dann said there had been extensive preparations with the travel industry, but warned “we would expect operators to make any adaptions that could lead to some changes in schedules” to ensure smooth passenger flow.

Some Border Force officers are expected to report for work alongside the Army and civil service personnel drafted in to assist, but Dann said “we simply will not know” how many would turn up each day.

The timing of the strikes also mean contingency plans could be upset by weather disruption. If so, normal contingency plans – including possibly keeping passengers on aircraft – will apply.

Another factor will be how many passengers arrive requiring extra scrutiny at immigration desks. Border Force personnel are normally given six weeks’ training, but stand-in staff have had only a few days.

Border Force is keen to stress Army and volunteer personnel have had sufficient training for the role they are expected to carry out during the dispute.