The 11:30am sailing from Belfast to Cairnryan has been unable to dock due to poor weather conditions and a generator failure.
It is understood the Stena Superfast VII has anchored off Cairnryan whilst engineers fix the generator and when completed it will attempt to dock within the next hour.
A passenger on board said: "The captain just announced that it is too dangerous to dock. We have been told to stay seated as a safety precaution. It was a rough crossing... things falling off shop display!"