Rail union RMT has threatened strike action at Eurostar.
Members are to be balloted in a row over safety and conditions at work.
The union claimed that staff face “worsening working conditions with unreliable trains, poor service recovery and operational safety concerns”.
The RMT says money is not being invested in improving fleet reliability or ensuring safe and secure conditions at stations.
A timeline for any industrial action has not been disclosed.
RMT general secretary Eddie Dempsey said: “Eurostar is making billions in revenue, but frontline staff are being left to deal with unsafe conditions and the consequences of poor management decisions.
“Eurostar is prioritising the bottom line ahead of our members concerns around investment in safety and good conditions at work.
“If Eurostar does not change course, strike action cannot be ruled out.”
A Eurostar spokesperson said: “The safety of our team members and customers is our absolute priority, and we continue to discuss the matter with RMT to resolve their concerns. No strike action is confirmed at this stage.”