British prime minister David Cameron has joined the chorus of criticism over industrial action that is expected to bring the UK to a standstill on Thursday.

He told the Commons there was no case for strikes, as government plans were “fair to taxpayers” and the public sector.

Addressing Parliament during prime minister’s questions, he said: “I don’t believe there is any case for industrial action tomorrow, not least because talks are still on-going.”

“It’s only a minority of unions who have taken the decision to go ahead and strike.”

Travellers have been warned to expect delays on arrival at UK ports and airports on Thursday as hundreds of immigration and customs officers are due to join the public sector strike.

Up to half a million air passengers flying into the UK 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.

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 industrial action is expected to affect travellers arriving in the UK because they are met by UKBA staff.

However, people leaving the UK will not be affected because departing passengers come into contact with security staff, employed by airport operator BAA, who will not be taking industrial action.

BAA, which runs Heathrow, Stansted, Edinburgh, Glasgow, Aberdeen and Southampton airports, said: “We have been informed by the UK Border Agency that arriving passengers should expect delays at immigration on Thursday as a result of industrial action by immigration officers who are members of the PCS union.

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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