More than 100 ferry staff in Hampshire are to take industrial action after talks over start times broke down.
Wightlink's Rail Maritime and Transport (RMT) union members will refuse to start work before 0530 GMT from Monday.
They have accused the firm of imposing earlier start times of 0500 GMT, without agreeing the change with them.
A spokeswoman for Wightlink said the firm was continuing discussions with the RMT to resolve the dispute as soon as possible.
The union said the industrial action will affect early sailings on the Portsmouth-to-Fishbourne route. It may also indirectly affect later trips.