Passengers flying with Jet2 and easyJet this week have been issued warnings as Border Force workers go on strike.
The two budget airlines have warned their customers they may face longer waits at airports impacted by the strikes, which will primarily hit immigration check-points.
As many a 100,000 members of the Public and Commercial Services union are walking out today and tomorrow in another long-running dispute, over pay, jobs and conditions.
Those taking part work for government departments including the Border Force, which work in airports and ports across the country.
The PCS is seeking a pay rise of 10% after the government offered 2%, which the union argued was well below the soaring rate of inflation.
Travellers entering the UK on February 1 could face long queues at airports during the strike, which runs until early tomorrow morning.
Yesterday a further Border Force strike was announced for half-term, running from February 13 to 17.
A spokesperson for Jet2 said: "We just wanted to remind you that there are planned Border Force strikes taking place from Wednesday 1st February until Thursday 2nd February 2023 at 07:00 . All our flights are planned to operate as normal.
"Things may take a little longer than usual, particularly when you arrive in the UK. Our teams will be on hand to help your journey run smoothly.
"You don't need to take any action, we will continue to update you if there are any further changes or updates."
EasyJet also updated its travel guidelines for the week.
A spokesperson said: "On February 1 strikes are taking place in the UK that may affect your journey.
"When you arrive passport queues may be longer than usual."
The spokesperson recommended that customers make sure their phones are charged and that they have all their medication, in case of a long wait.
They added: "If you’re usually able to travel with either an ID or a passport, we’d recommend that you bring your passport as these can be used at any airport with eGates in place and will help to speed up your journey."
A statement from the government on the Border Force strikes read: "The proposed strike action will impact international arrivals at all UK air and maritime ports as well as those travelling to the UK from UK border controls in Calais, Dunkirk and Coquelles in Northern France.
"If you are travelling into the UK via any entry point, you should prepare for disruption and check before you travel.
"Our number one priority is to keep our borders safe and secure, and we will never compromise on this."
The impact on airports appears to be minimal so far, with Heathrow reporting minor queues in its immigration halls this morning.
A spokesperson for the airport said: “Heathrow is fully operational, passengers are flowing through the Border smoothly with Border Force and the military contingency providing a good level of service for arriving passengers.
"We are working to support Border Force’s plans to continue the smooth operation of the airport during this period of industrial action.”