Last-ditch talks are being held in a bid to avert a 48-hour strike by drivers and on-board staff on Eurostar trains.
About 70 drivers and 20 on-board staff are planning to walk out on Friday and Saturday in a row over pay, threatening disruption to services between London, Paris and Brussels.
The drivers' union Aslef and the Transport Salaried Staffs Association 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 major disruption to services if the strike goes ahead.
Eurostar and Aslef both said they were confident that an agreement will be reached during the talks and that industrial action will be averted.
