The Pride of Kent is one of eight ships to need inspections before re-entering service after 800 staff were sacked.
The firm replaced staff with agency workers paid less than the minimum wage, but the government says it has prepared measures to block P&O’s plans.
On Friday, another P&O ferry was held after being declared “unfit to sail”.
The MCA was inspecting the Pride of Kent to make sure it was safe to go to sea without passengers or cargo.
A spokesperson for the agency said: “Our surveyors are in the process of detaining the Pride of Kent. We are awaiting confirmation of all the detainable items.”
P&O Ferries did not immediately respond to a request for comment.
Another P&O Ferries vessel, the European Causeway, failed an MCA Port State Control inspection last week.
The ferry was detained in Larne over “failures on crew familiarisation, vessel documentation and crew training.
Port State Control is the inspection of foreign ships in national ports, to check that the condition of the ship and its equipment comply with the requirements of international rules – including emergency procedures such as firefighting and evacuating the ship.
The RMT union said: “It’s rare enough for the MCA to impound a ferry but P&O have now had two in a week after the jobs carve-up which speaks volumes about the dire state of their operation”.
Transport Secretary Grant Shapps tweeted that “safety would not be compromised”.
