The head of the union representing striking Border Force staff has warned that industrial action could last for six months.
Mark Serwotka, general secretary of the Public and Commercial Services (PCS) union, issued the warning as industrial action started at six UK airports including Heathrow, Gatwick and Manchester.
The walkouts will affect border checks from today (Friday) to Boxing Day and then between December 28 and 31.
The action is also affecting Birmingham, Cardiff and Glasgow airports together with the port of Newhaven.
The Border Force strikes could affect a total of 8,910 flight arrivals – equating to more than 1.7 million seats, according to aviation analytics firm Cirium.
Today (Friday) is expected to be the busiest day for arriving flights at Birmingham, Glasgow and Gatwick, while December 30 will be the busiest at Manchester.
Heathrow and Cardiff will see an equal number of arrivals on December 23 and December 30.
Military personnel and civil servants have been drafted in to cover passport checks for arriving passengers.
However, Serwotka told BBC Radio 4’s Today programme the union “can sustain this strike action for months,” with a strike mandate in place until May.
He said: ”I think that not only could there be six months [of strike action], I think in January what you’ll 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.”
Speaking about the striking workers he represents, Serwotka said many are “the in-work poor” who need more than a 2% pay rise – which is what the government has offered.
Strikes could last six months if the government does not “get around the negotiating table,” he said.
Heathrow said: “Though the vast majority of journeys will be unaffected, passengers arriving at Heathrow on strike days who are not eligible to use eGates may experience longer wait times at border control.
“Heathrow is in constant contact with Border Force and the Home Office to strengthen contingency plans and minimise any disruption.
“We are also working closely with our partners and airlines to protect travel plans and get our passengers where they want to be this Christmas.”
British Airways said in an update on its website: “Due to industrial action by UK Border Force officers, we are expecting to see longer waiting times to pass through the UK border at a number of UK airports.
“This industrial action, which is affecting a number of UK airports, including London Heathrow and London Gatwick, is likely to lead to longer waiting times to pass through the UK Border, for customers ending their journey in London or connecting to another destination in the UK and Ireland.
“We’ll do everything we can to ensure journeys runs smoothly.”
Which? Travel editor Guy Hobbs said: “If your flight is cancelled or delayed as a result of strike action by Border Force staff, you are unlikely to be eligible for compensation as these events are considered outside of the airline’s control.
“However, your airline still has a duty of care to you – and if your flight is cancelled you have a right to a refund or to be re-routed as soon as possible, even if that means flying with a rival carrier.”