The European Causeway, which can carry up to 410 passengers, is currently adrift around five miles off Larne Harbour after it “lost all power”, the RMT union said.
It left Cairnryan at midday with an unknown number of passengers onboard.
The ferry was due to arrive at Larne Harbour at 2pm but never arrived, prompting the RNLI to launch three lifeboats to attend the scene.
It is understood that the boat lost power as it attempted to embark on its voyage.
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