British drivers and onboard staff on Eurostar trains will strike on Friday and Saturday in a pay dispute, train drivers' union ASLEF announced on Wednesday.
"Talks between ASLEF and Eurostar concerning international allowances have broken down this afternoon and industrial action will be taking place on Friday and Saturday," ASLEF said in a statement.
"The problem has arisen because the real value of drivers' international allowances has declined as the value of the pound sterling has dropped compared with the euro."
The union added that allowances had increased in line with British inflation but had not mirrored exchange rate changes.
About 70 drivers and 20 onboard staff will stage a walkout that threatens disruption on train services between Brussels, London and Paris.
However, Eurostar expressed confidence that it would run a normal service despite the strike, adding it had made a "fair and reasonable" offer over staff allowances.
"We are confident that we will cover the trains with our French and Belgian drivers and will operate a normal service," a Eurostar spokesman said.
ASLEF and the Transport Salaried Staffs Association (TSSA) have meanwhile warned of further industrial action on December 26 and 27 - which are also very popular dates for holiday travellers.
"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," added TSSA general secretary Gerry Doherty.
"But their anger would be better directed at Eurostar who have been refusing for months to reach a sensible settlement with us."Reuse content