Two P&O Ferries vessels are 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 European Highlander on Thursday and Norbay on Wednesday.
P&O Ferries suspended most of its sailings after replacing 786 seafarers with cheaper agency staff on March 17.
European Highlander normally operates between Larne in Northern Ireland and Cairnryan in Scotland, while Norbay serves the Liverpool-Dublin route.
A total of eight P&O Ferries vessels will be examined by the MCA through the Port State Control regime.
A spokesman for the agency said: “Our surveyors are carrying out a full inspection of the P&O ferry, European Highlander, 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.
“The inspection of Norbay is ongoing.
“There are no further inspections of P&O Ferries at the moment but we will reinspect 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 was detained after an initial inspection on March 25 uncovered 31 safety failings.
The ship was cleared to resume serving the Larne-Cairnryan route a fortnight later following another examination.
Pride Of Hull passed its inspection, enabling it to operate between Hull and Rotterdam.
Two other Dover-Calais ferries – Pride of Canterbury and Spirit of France – are out of action as they have not been fully inspected.
Safety fears were raised after P&O Ferries replaced nearly 800 seafarers with cheaper agency staff on March 17.
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”.