Border Force workers at several major British airports including the country's busiest, Heathrow, will go on strike for eight days this month in a dispute over pay, threatening disruption to Christmas travel.
The PCS trade union said staff employed by Britain's interior ministry in passport booths would take action at London's Heathrow and Gatwick airports and a handful of regional ones, plus the Port of Newhaven.
The union's general secretary, Mark Serwotka, said around 2,000-3,000 staff would be involved in the walkouts on every day but one between 23-31 December.
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Asked why the union had picked the Christmas period, Serwotka said, "Those people will lose pay when they go on strike, they do it as a last resort and the job of the union is to ensure that the action that we call is noticed."
Immigration minister Robert Jenrick said the decision was "unjustifiable" and would ruin the plans of thousands of families and businesses.
"While we are working closely with all UK ports and airports and have robust plans in place to minimise any delays if strike action goes ahead, passengers should be prepared for their plans to be severely disrupted," he said in a statement.
Aviation analytics firm Cirium estimated more than 10,000 flights, potentially carrying more than two million people, were due to arrive at the affected airports between 23 and 31 December.
Travellers in Britain already face major rail disruption over the Christmas period, with more than 40,000 railway workers due to walk out on 13-14, 16-17 and 24-27 December, and 3-4 and 6-7 January.
A Heathrow spokesperson said the airport was working with airlines and Border Force on plans to mitigate disruption.
Gatwick said it expected flights to operate as normal and it would also make extra airport staff available to help passengers on strike days.