The BBC, quoting the Maritime and Coastguard Agency (MCA), said the Pride of Kent was one of eight ships to need inspection before re-entering service.
The MCA was said to be inspecting the ship to make sure it was safe to sail without passengers or cargo.
A spokesperson for the agency told the BBC: "Our surveyors are in the process of detaining the Pride of Kent. We are awaiting confirmation of all the detainable items."
https://twitter.com/grantshapps/status/1508522250227662854
Transport secretary Grant Shapps tweeted: "The MCA have informed me tonight that they have carried out an inspection on a ship belonging to P&O Ferries.
"As a result, the Pride of Kent ship has now been detained. Safety will not be compromised and further checks will continue."
Another P&O Ferries vessel, the European Causeway, was detained in Larne last week after failing an inspection.
It comes after P&O Ferries sparked outrage by replacing all 800 crew with agency workers paid less than the minimum wage.
