“During the periods of industrial action, travellers should be prepared for disruption,” chief operating officer Steve Dann said.
Heathrow, Britain’s busiest airport, reported “minimal queuing” in its arrivals halls.
“Immigration halls are free flowing … with Border Force and the military contingency providing a good service,” a spokesperson said.
Gatwick, Britain’s second busiest, said passengers should expect longer wait times at passport control between Friday and the end of the year.
“We … anticipate some disruption, but flights are operating normally, arrivals and departures, and we expect that to continue,” Adam Jones, head of passenger operations, told Sky News.
Ahead of the strike action, which is scheduled to run up to Dec 31 excluding Dec 27, the government said it was training members of the armed forces and government officials to help staff passport controls at airports and ports.