A ship operated by P&O Ferries has been detained in Larne for being "unfit to sail".
The European Causeway vessel has been held in the Northern Ireland port due to "failures on crew familiarisation, vessel documentation and crew training", the Maritime and Coastguard Agency (MCA) said.
Britain's transport minister Grant Shapps said he will not compromise the safety of P&O vessels and insisted that the company will not be able to rush training for inexperienced people.
The company has sacked almost 800 seafarers and plans to replace them with agency staff on cheaper salaries.
A spokesperson for the Maritime and Coastguard Agency 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."
There were no passengers or freight on board the P&O European Causeway vessel when it was detained in Larne, the agency said.
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."
The detention of ships is based on concerns over their safety and to prevent them going to sea.
The European Causeway entered service in 2000 replacing the Pride of Rathlin, according to the P&O Ferries website.
"Specifically designed for our Cairnryan to Larne route, she has not operated elsewhere and has only been taken out of service if she needs a refit," the website said.
The RMT union said it welcomed the detention of the European Causeway and it demanded the UK government "seize the entire fleet" of P&O vessels.
Earlier, Mr Shapps said the boss of P&O Ferries should resign after his "brazen" and "breathtaking" comments about "knowingly breaking the law".
Mr Schapps said: "I thought what the boss of P&O said yesterday about knowingly breaking the law was brazen and breathtaking, and showed incredible arrogance.
"I cannot believe that he can stay in that role having admitted to deliberately go out and use a loophole - well, break the law, but also use a loophole."
Pressed on whether that meant he was calling for Mr Hebblethwaite to resign "right now", he said: "Yes."
The chief executive admitted that the new crews are being paid below the UK's minimum wage apart from on domestic routes, but insisted this is allowed under international maritime rules.
Mr Hebblethwaite, whose basic annual salary is £325,000, revealed that the average hourly pay of the new crew is only £5.50.
The minimum wage in the UK for people aged 23 and above is £8.91 per hour.