An escalating row between Irish Ferries and SIPTU resulted in the cancellation yesterday of a scheduled sailing from Cherbourg to Rosslare.
The union accused the company of leaving 200 passengers who were due to sail last night "stranded" at the French port.
A company spokesman, however, said about 60 passengers were affected. They were being rerouted on other services, and compensated for any loss.
Two other scheduled return trips on the same route, which had been due to take place before the service was suspended at the end of this month, have been cancelled and passengers notified.
The company's action was taken on foot of SIPTU's decision to serve notice of industrial action on Irish Ferries services from next Friday.
The union is opposed to a move to outsource employment on the Rosslare-France service when it resumes in March.
SIPTU official Mr Paul Smyth said the decision to cancel the service yesterday amounted to a "lock-out", and said it would "further inflame the situation".
A company spokesman, however, said it did not want to risk leaving the service in place up to the time when industrial action, as yet unspecified, was due to commence. Instead, the company had withdrawn the service when bookings were particularly low.
SIPTU says it has yet to decide on the nature of the industrial action to take place. The company says if it is unable to offer certainty to customers, it may respond by suspending services.