A P&O ferry has been detained at Larne Port over safety concerns by the Maritime and Coastguard Agency.
The European Causeway will not be able to leave the port until issues around training, crew familiarisation and vessel documentation are resolved, the agency said.
It said, "We can confirm that the European Causeway has been detained in Larne.
"It has been detained due to failures on crew familiarisation, vessel documentation and crew training.
"The vessel will remain under detention until all these issues are resolved by P&O Ferries. Only then will it be reinspected."
Transport Secretary Grant Shapps said the ferry has been detained for being "unfit to sail" following reports a vessel had been impounded at the port of Larne in Northern Ireland.
Mr Shapps tweeted: "Following my instruction to inspect all P&O vessels prior to entering back into service, the @MCA--Media has detained a ship for being unfit to sail.
"I will not compromise the safety of these vessels and P&O will not be able to rush inexperienced crew through training."