Security guards have started strike action at the Eurostar terminals in London and Ashford in Kent.
Workers from security firm Chubb are taking action today and on Saturday in a dispute over pay. RMT union members are walking out for three-and-a-half hours on each day at different times at the two sites.
Eurostar has said it does not expect the planned strike action to affect bank holiday weekend services and that it has contingency plans in place.
The RMT said it was concerned at the company's view that a lack of security guards would have little effect.
Bob Crow, general secretary of the RMT, said: "I have written to Eurostar expressing our concern that the use of staff who are untrained or who do not have the required counter-terrorism check would seriously compromise its passenger-screening operation.”
Talks took place during the week in an attempt to avert strike action, but no compromise was reached. The union said Chubb had offered workers a 3% pay rise.