The second P&O ferry has failed a safety inspection and is in the process of being taken into custody by theMCA.
The Pride of Kent is one of eight ships that need to have their engines checked.
The firm replaced staff with agency workers who were paid less than the minimum wage, but the government says it has prepared measures to stop P&O.
Another P&O ferry was held on Friday after it was declared not fit to sail.
The Pride of Kent was being inspected to make sure it was safe to leave without passengers or cargo.
The Pride of Kent is being held by the agency. We are waiting for confirmation of all the detainable items.
A full Port State Control inspection is required for the ferry to carry passengers and freight again.
Port State Control is the inspection of foreign ships in national ports to make sure that they comply with international rules, such as emergency procedures, and that the condition of the ship and its equipment complies.
The European Causeway ferry failed an inspection last week. The ferry was held in Larne for failing to comply with crew familiarisation, vessel documentation and crew training.
It is rare for the MCA to take away a ferry, but P&O have had two in a week after the jobs carve-up, which speaks volumes about the dire state of their operation.
Transport Secretary Grant Shapps said that safety wouldn't be compromised.
UPDATE ⚠️ The @MCA_media have informed me tonight that they have carried out an inspection on a ship belonging to P&O Ferries.
As a result, the #PrideOfKent ship has now been detained.
Safety will not be compromised & further checks will continue.