Travellers should expect disruption at the borders due to a strike, a Border Force boss has said.
Chief operating officer Steve Dann said there were “robust” plans in place to keep “most if not all” ports open.
Around 1,000 Border Force staff who are members of the Public and Commercial Services Union will strike every day from Friday to the end of the year, with the exception of December 27.
The walkout over pay and pensions affects Heathrow, Birmingham, Cardiff, Gatwick, Glasgow and Manchester airports, and the port of Newhaven in East Sussex.
Mr Dann said: “Border Force’s number one priority is to keep our borders safe and secure, that’s something that we will not compromise on. It’s non-negotiable.
"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.
“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.”
The industrial action could lead to delays in checking passports, meaning long queues or even people being held on planes.
Military personnel and volunteers from the Civil Service are being drafted in to fill workplace gaps.
Mr Dann said: "Military personnel and volunteers from across the Civil Service have been trained to support Border Force at airports and ports through the strike period.
"We have a reasonable expectation that by using this contingency workforce, we will be able to deliver a service that keeps most if not all ports open."
He went on: "Our contingency workforce will not be able to operate with the same efficiency as our permanent workforce.
“As in the case with any industrial action, we simply will not know the levels of Border Force permanent officers who will report for duty each day and what the operational impacts might be."
Travellers are being urged to use e-gates if they can, check with operators that there has been no change to their service, and to be "kind and patient" with staff at Border Force.