Strikes by Border Force staff at UK airports could go on for months unless the government enters talks over pay, the head of the PCS union has said.
Mark Serwotka said the union had a "mandate" for walkouts up until May.
Prime Minister Rishi Sunak said he was "sad" about disruption caused by strikes, but said he had acted "fairly and reasonably" over public sector pay.
Thousands of travellers arriving in the UK had been told to expect delays but so far disruption has been minimal.
On the roads, however, there was "severe congestion" in some areas, according to the AA as the Christmas getaway gathered pace.
It said that rail strikes, which are due to restart on Saturday, had added to the larger number of cars on the road, while accidents on the M1 and a partial closure of the M25 due to flooding had caused major traffic jams.
Around 1,000 Border Force staff – many of whom check passports – are staging the first of a series of strikes from Friday to 26 December and from 28 to 31 December.
Employees are walking out at Heathrow, Gatwick, Manchester, Birmingham, Cardiff and Glasgow airports, as well as the Port of Newhaven. Military personnel and civil servants have been drafted in to cover strikers.
A spokesperson for Heathrow Airport said on Friday afternoon that operations were going "smoothly" and the airport is running as normal.
"The Immigration halls are free flowing with Border Force and the military contingency providing a good level of service for arriving passengers," the spokesperson added.
Glasgow Airport also experienced "no issues" resulting from the strike action, a spokesperson told the BBC. There have been no reports of delays in the other airports either.
Members of the UK Armed Forces who provide cover for striking public service workers during the Christmas period will receive extra bonus payments for every day they work, the Defence Secretary Ben Wallace has announced.
The Ministry of Defence said each stand-in worker would get a £20 bonus for every day they spend training or deployed during the festive period.
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Union boss says Border Force strikes can continue for "many, many months"
Mr Serwotka said that any disruption for passengers was an "unfortunate reality" of the strikes but said any anger should be directed at the government, who he claimed had "ignored" the union.
He said the union was raising cash for a strike fund which meant members could "sustain" strikes "for months and after Christmas".
"Not only could it be six months, I think in January what you will see is a huge escalation of this action in the civil service and across the rest of the economy unless the government get around the negotiating table," he said.
Mr Sunak said: "I want to make sure we reduce inflation, part of that is being responsible when it comes to setting public sector pay.
"In the long term it's the right thing for the whole country that we beat inflation."
Separately, a planned 72-hour walkout by Menzies baggage handlers at Heathrow that had been due to start on 29 December has been called off after members of the Unite union voted to accept an improved pay offer.
Here is a full guide to flight cancellation rights.
Jasmine O'Donoghue, 25, has been in Costa Rica since 16 November and is due to travel to Heathrow then on to Jersey on 27 December, which is not a strike date.
Nevertheless, she has been advised she should change her flight due to the impact of the strikes on domestic transfers.
"Right now I don't know if I'm getting on the flight, or will change my flight," she said. "It would be nice for my family and my boyfriend if I was at home for New Year after being away for so long."
Aviation data firm Cirium said over the period of the festive strikes, a total of 8,910 flights will arrive, with a capacity of nearly 1.8 million people.
Steve Dann, Border Force chief operating officer, said military personnel and civil servants, "many of whom are sacrificing their Christmases", would "not be able to operate with the same efficiency as our permanent workforce".
Border Force is the latest sector to stage industrial action over pay, jobs and conditions. Other walkouts that are scheduled over the next few days include:
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