Steve Dann, chief operating officer, said he hoped to keep all airports and ports open as about 1,000 Border Force workers walk out over pay.

Staff will strike on 23-26 and 28-31 December affecting arrivals to the UK.

Heathrow, Gatwick, Glasgow, Manchester, Birmingham and Cardiff airports will be affected.

Staff at the port of Newhaven will also strike. All strikers are members of the Public and Commercial Services Union (PCS) which balloted for strike action after it said the government had refused to increase a 2% pay rise offer.

"In anticipation of the strike action, Border Force has for a number of months undertaken extensive planning, and we've been working with the travel industry and continue to work closely with all UK ports to assess the impacts of the announcement on the travelling public," Mr Dann said.

"We do have robust plans in place to minimise delays to passengers, but we've been very clear from the start that people should be prepared for disruption and take action to plan ahead."

Earlie this month, Home Secretary Suella Braverman said people should "think carefully" about their travel plans on strike days.

Military personnel, civil servants and Home Office volunteers have been trained to check passports when Border Force staff walk out.

But there are fears that delays in checking the passports of arriving passengers could lead to long queues and even people being held on planes, disrupting subsequent departures.

The PCS has said the replacement workers in have not been given sufficient training.

The union said its officers get five weeks training to do their jobs yet many of the stand-in staff have been given one week.

And it claimed civil servants from the National Crime Agency had been "trained at the last minute", with one day's training.

"You can't seriously expect people who have been trained for a day, or even a week, to do the job of officers with five weeks' training and many years experience," a PCS spokesperson said.

But Border Force managers believe they can maintain the same standards as normal for passport checks, and Mr Dann said safety and security at borders will be "non-negotiable".

Officials have refused to disclose the number of replacement staff or the length of training they have received.

Christmas break affected

It is the first Christmas since 2019 that airlines have been able to operate without Covid restrictions and up to two million passengers are expected to arrive between 23 and 31 December at the airports where the strikes will take place, according to aviation analytics firm Cirium.

It has previously said more than 10,000 flights are scheduled to arrive at Heathrow, London Gatwick, Manchester, Birmingham, Cardiff and Glasgow airports during the period.

Travel expert and commentator Simon Calder said he was concerned about whether staff will be able to cope with the rush of early morning passengers at Heathrow airport on Friday, as the Christmas getaway begins.

Mr Dann urged passengers to use the automated electronic passport gates at airports if possible.

The BBC understands that the Border Force asked airports that are expecting passenger numbers on strike days to be above 70-80% of 2019 levels to "supress demand".

The move led British Airways and Virgin Atlantic to stop selling new tickets for inbound flights to Heathrow on the strike days.