Another P&O Ferries vessel is being inspected as the company attempts to resume normal operations after sacking nearly 800 seafarers.
The Maritime and Coastguard Agency (MCA) said it began assessing Norbay on Wednesday.
P&O Ferries is hoping the ship can restart sailings on the Liverpool to Dublin route.
Eight of its vessels will be examined by the MCA following the mass sackings.
A spokesman for the agency said: "Our surveyors are carrying out a full inspection of the P&O ferry Norbay before it returns to service, to make sure it complies with international regulations on manning and safe operation, in particular emergency procedures such as firefighting and evacuating the ship.
"There are no further inspections of P&O Ferries at the moment but we will re-inspect when requested by P&O Ferries."
The ferries Spirit Of Britain and Pride Of Kent remain under detention after safety issues were found.
This caused a shortage of ferry capacity in the run-up to Easter on the key Dover-Calais route, which contributed to large queues of lorries on coastbound roads in Kent.
European Causeway - which runs between Larne in Northern Ireland and Cairnryan in Scotland - was detained after an initial inspection on 25 March uncovered 31 safety failings.
The ship was cleared to sail a fortnight later following another examination.
Pride Of Hull passed its inspection, enabling it to operate between Hull and Rotterdam.
Three other ferries are out of action as they have not yet been fully inspected.
Safety fears were raised after P&O Ferries replaced nearly 800 seafarers with cheaper agency staff on 17 March.
The firm suspended most of its sailings but reportedly only expected the disruption to last up to 10 days.
On Tuesday, shadow transport secretary Louise Haigh accused the company of being a "rogue operator" which is "cutting corners and putting key UK shipping routes at risk".
P&O Ferries said in a statement that "any suggestion" that safety is being compromised is "categorically false".