An inability to safely deploy lifeboats or life rafts was one of 31 failures discovered on a P&O Ferries vessel, according to a new report.
Inspectors for the Maritime and Coastguard Agency (MCA) found that the “launching arrangements for survival craft” on European Causeway were “not as required”.
The ship has two lifeboats and several life rafts for use in emergencies.
P&O Ferries was hoping to be in operation again by the Easter weekend, but it had its vessel the Pride of Kent detained for a second time after a re-inspection found deficiencies including in its 'safety systems and crew documentation'.
Among other flaws identified by the MCA were an inflatable evacuation slide not properly maintained, inadequate fire prevention systems and crew having a lack of familiarity with radio equipment.
There were also problems with labour conditions, navigation and documentation.
The Paris Memorandum of Understanding (MOU) – an alliance of 27 national maritime authorities, including the UK – listed the 31 safety deficiencies but did not provide further details.
Analysis by the PA news agency revealed more failures were found than in any of the other 46,000 Port State Control inspections of ships within the Paris MOU in the past three years.
P&O Ferries was widely condemned after sacking nearly 800 seafarers without notice on March 17 and replacing them with cheaper agency staff.
The firm suspended most of its sailings, including by European Causeway on the Northern Ireland-Scotland route.
It reportedly predicted the disruption would only last around 10 days, but European Causeway was detained at Larne, Northern Ireland by the MCA on March 25.
The 22-year-old ship, which can carry up to 410 passengers, was not cleared to sail until April 8 following another examination.
Spirit of Britain and Pride of Kent remain under detention after safety issues were found.
The Pride of Kent has been stuck in the Port of Dover since the company sacked 800 of its staff on March 17.
This has caused a shortage of ferry capacity on the key Dover-Calais route, contributing to large queues of lorries on coastbound roads in Kent.
Pride of Hull passed its inspection, enabling it to operate between Hull and Rotterdam.
Four other ferries are also out of action as they have not yet been inspected.
A spokesman for P&O Ferries said: “Following confirmation from the MCA that the European Causeway is safe to sail, we were delighted to restart sailings between Larne and Cairnryan last week.
“P&O is pleased to play its part in supporting customers to move vital goods between Northern Ireland and Scotland.
“It is clear that inspections of our vessels have reached an unprecedented level of rigour.
“We welcome this additional scrutiny and would reiterate that the safety of our passengers and crew is our foremost priority.
“Any suggestion that it is being compromised in any way is categorically false and we look forward to all of our ships welcoming tourist passengers and freight customers again as soon as all mandatory safety tests have been passed.”
The MCA has said its inspections are conducted “in exactly the same robust way” for every ship.