Eurostar services could be severely disrupted this month as staff threaten to strike in a row over lone working and alleged victimisation of a staff member.
Rail union RMT confirmed it will begin balloting Eurostar train managers based at St. Pancras with plans to walkout from March 25.
Eurostar said it was in discussion with the union and hoped to find a solution.
The ballot, for both strike action and action short of a strike, will open on Tuesday and close on 15 March.
The cross channel operator faces the action after a train manager was charged with misconduct after objecting to work a section of the route on her own.
RMT General Secretary Mick Cash said: “The outrageous decision taken by Eurostar management to discipline our member in this manner is totally unacceptable.
“This is a situation that the union will not allow to go unchallenged. Our members recognise full well that it could be them next and that it is imperative that they stand together.”