Stena Line officers could be balloted over strike action in a row over UK jobs.
The Nautilus International union wants the Swedish ferry owner to enter into talks about the withdrawal of the 16,300-gt UK-flagged freight ro-ro Capucine (built 2011).
The ship and a Dutch-flagged ro-pax are due to be replaced by two other Dutch-registered vessels, one of which is chartered in, when their bareboat deals expire in January. The first will have Dutch crew and the second Dutch officers.
Members at the union's UK conference voted on Tuesday for an emergency motion voicing concern over the plans and backing potential industrial action.
"This against a backdrop of UK seafarers serving on this route for more than 15 years, far longer than any other nationality," the motion noted.
Nautilus wants equal treatment for UK and Dutch employees.
If no talks take place, the union will "consider that a dispute situation has arisen and ballot its members for industrial action in addition to pursuing all appropriate legal avenues".
Nautilus general secretary Mark Dickinson said: "This is an extremely serious situation and our members have made it very clear that they will not tolerate the company playing one side of the North Sea seafarer workforce against the other side.
"Equal treatment should prevail on both sides of the seaway, and the company must beware of the risks it runs by failing to treat its UK staff fairly."
Comment has been sought from Stena Line.
