Air travellers were told there could be months of disruption unless the UK government improves a pay offer, the union leader for the striking staff said.
Mark Serwotka, general secretary of the Public and Commercial Services (PCS) union, said there would be a “huge escalation” in industrial action in January across the civil service unless ministers enter into negotiations.
The strike among Border Force staff who work at passport control desks, will take place every day for the rest of the year except December 27, with an estimated 1.8 million people arrive at UK airports.
On Friday, airports were largely able to deal with the first day of the strike with help from the military.
About 1,000 PCS staff employed by the Home Office have walked out at Heathrow, Birmingham, Cardiff, Gatwick, Glasgow and Manchester airports, as well as the Port of Newhaven in East Sussex.
Travellers were told to expect delays and managers feared queues could lead to people being held on planes, disrupting subsequent departures.
But airports got through day one without major problems.
“Operations continue to run smoothly and the airport is operating as normal,” a Heathrow representative said.
“The immigration halls are free flowing with Border Force and the military contingency providing a good level of service for arriving passengers.”
“Everything is going OK at the moment. There’s plenty of staff. The e-gates are all operating. It’s going well,” a spokesman for Gatwick said.
Mr Serwotka said some travellers were simply being “waved through” airports.
“The word we have from these airports is people were being waved through, passports were not being checked properly, because the work our members do is highly skilled and that cannot be replicated by an army personnel,” he said.
He added: “Our strike mandate lasts right up until May. We will be supporting this action up to May and we would re-ballot again if we have to.
“I think in January what you will see is a huge escalation of this action in the civil service and across the rest of our economy unless the government get around the negotiating table.”