Last-ditch talks aimed at averting a 48-hour strike by drivers and on-board staff on Eurostar trains have broken down without agreement.
Around 70 drivers and 20 on-board staff will now walk out on Friday and Saturday in a row over pay, threatening disruption to services between London, Paris and Brussels.
Eurostar said it was confident of running a normal service despite the industrial action.
The drivers' union Aslef and the Transport Salaried Staffs Association (TSSA) have warned they will take further strike action on Boxing Day and December 27 unless the row is resolved.
Gerry Doherty, leader of the TSSA, said: "We know this will cause huge disappointment to travellers who are planning Christmas shopping trips to Lille and Paris and I wish to apologise in advance for the inconvenience this will cause.
"But their anger would be better directed at Eurostar, who have been refusing for months to reach a sensible settlement with us."
Eurostar is operated by drivers from France and Belgium as well as the UK but the unions believe there will be disruption to services.
The dispute is over allowances to UK employees when they are in Paris and Brussels. Eurostar said it made a "fair and reasonable" offer, but a meeting between the two sides broke down.
"We are confident that we will cover the trains with our French and Belgian drivers and will operate a normal service," said a spokesman.
