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 some "severe congestion" in some areas according to the AA motoring organisation as the Christmas getaway gathered pace. It said that rail strikes, which are due to restart tomorrow, had added to the larger number of cars on the road.
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.
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.
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.
