P&O's Spirit of Britain has failed safety checks after inspectors found 'serious deficiencies' with the ship.
The Maritime and Coastguard Agency (MCA) carried out a general inspection of the vessel, currently berthed at Dover port, yesterday and surveyors decided to detain it under Port State Control regulations.
Inspectors embarked the ship at the ferry firm's request to see if it could be cleared for sail, but now P&O must put right the deficiencies before it can be reinspected and brought back into service.
A spokesman for the MCA said: “The Spirit of Britain has been detained due to surveyors identifying a number of deficiencies which were grounds for detention.
“We have advised P&O to invite us back once they have addressed the issues. We do not know yet when this will be.”
Safety inspections are necessary after P&O sacked nearly 800 of its workforce on March 17 in a shock move with no prior consultation or warning.
It replaced the experienced seafarers with agency staff, being paid a lower wage.
The firm said it had no choice but to lay off staff and replace them in order to save it from going under in a move that has been condemned by the government, trade unions and public.
No P&O ships have sailed from Dover since the mass firing, which sparked protests and calls for the company to be boycotted.
The MCA has already carried out an inspection of the Pride of Kent, which also sails the Dover to Calais route, which it failed.
The ship remains under detention.
The Pride of Hull, which runs the Hull to Rotterdam route, and European Causeway, Larne to Cairnryan, have been inspected and cleared to sail.
A P&O Ferries spokesman said: “The Spirit of Britain will remain berthed in its current port, following inspections by the Maritime & Coastguard Authority (MCA).
"In the past few days, both the European Causeway and the Pride of Hull have been deemed safe to sail by the MCA, and we continue to work with all relevant authorities to return all our ships to service.
"We take the safety of our passengers and crew very seriously and look forward to all of our ships welcoming tourist passengers and freight customers again as soon as all mandatory safety tests have been passed.”
Four other ferries are yet to be inspected, while the Pride of Kent will need to be reinspected before it can sail.
It comes as P&O Ferries said it was hoping to resume its services from Dover this week.
In a statement released last Wednesday, bosses at the firm said it would be preparing to 'resume services across a number of vital routes'.
But on Sunday, the firm confirmed that journeys on any of their vessels to France have been cancelled until Good Friday.
Formal criminal and civil investigations have been launched following the mass sacking, with the Insolvency Service looking into the actions of the ferry firm and whether it broke the law.
The suspended services have had a knock-on effect on Kent's roads, with delays at the port leading to Operation Brock being put into action and other road closures.