Up to half a million air passengers flying on Thursday could face severe disruption because of a strike by border officials at British airports.
At least seven out of 10 staff at the UK Border Agency are expected to walk out on Thursday.
The UKBA has warned that “some passengers may experience delays at the border”.
“Those passengers who can travel on an alternative day may therefore wish to do so,” it wrote in a statement.
The striking staff are members of the Public and Commercial Services Union (PCS).
Mark Serwotka, PCS general secretary, said: “We expect longer queues and for managers to waive controls they would usually undertake. There will be less presence in terms of people coming and going. There is a theoretical risk to security in terms of smuggling and security of the borders, which essentially could be weakened.”
The PCS voted earlier this month to join the strikes, with a 61.1% support for the walk-outs from a turn-out of 32.4%.
Serwotka said that UKBA attempts to draft in replacement staff to conduct passport checks would not prevent delays.
He said: “It is likely that there will be severe disruptions and delays affecting both ports and airports. Our membership in Dover and Heathrow is particularly strong.”
Around 500,000 passengers pass through UK airports every day, with around one-third of those at Heathrow.
Jonathan Sedgwick, acting chief executive of UKBA, said: “We will do everything we can to minimise disruption and inconvenience to travellers. But our priority will always be to ensure that the UK border remains secure.”
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