A second P&O ferry has been detained, the Maritime and Coastguard Agency has confirmed.
The Pride of Kent, based in Dover, has now been seized by officials following an inspection on Monday, 28 March.
A spokesperson for the Maritime and Coastguard Agency said: "The Pride of Kent has been detained due to failures on vessel documentation, crew familiarisation and training, and emergency equipment not functioning properly, indicating a failure of the implementation of a safety management system.
"We have advised P&O to invite us back once they have addressed the issues. We do not know yet when this will be."
The MCA carried out a general inspection of all aspects of the ship’s operation.
Surveyors found serious deficiencies and decided to detain it under Port State Control regulations.
Detaining a vessel under Port State Control means that the operator and the Flag State must put right the deficiencies before inviting the MCA to reinspect.
P&O Ferries wanted to conduct sea-trials with a view to putting the ship back into service by the end of the week.
ITV News Economics Editor Joel Hills described the news as "bleak" for the company.
"None of its eight ferries have ever been detained while owned and operated by the company, until four days ago," he reported.Two P&O ships have now been detained since Friday. The Pride of Kent has operated out of Dover since 1992.
The RMT Union has welcomed the MCA's detention of the vessel.
General Secretary Mick Lynch said: "The seizing of the Pride of Kent by the MCA this evening should be adequate evidence for the Government that the gangster capitalist outfit P&O are not fit and proper to run a safe service after the jobs massacre."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. "It's now high time for these important vessels to be taken over under public control with the sacked crews reinstated as the only way to get these crucial ferry routes back running safely. "
The MCA said that eight P&O Ferries need to be inspected so it is satisfied they are safe to go to sea.
The Pride of Hull has been inspected and was cleared to set sail.
The European Causeway (Larne – Cairnryan) and Pride of Kent (Dover – Calais) are currently under detention.
The European Highlander was cleared by inspection for relocation but still needs to have its Port State Control inspection.
Pride of Canterbury, Spirit of Britain and Spirit of France (Dover - Calais) and Norbay (Liverpool – Dublin) are yet to have their Port State Control inspections.
The boss of P&O Ferries last week insisted no criminal offence was committed when his firm sacked 800 workers without notice last week.
CEO Peter Hebblethwaite admitted to a committee of MPs that P&O broke employment law by firing hundreds of employees with no warning but in an email to employees he insisted "no criminal offence has been committed".
He said the mass sacking was "an incredibly difficult decision" but one that was "necessary and pivotal for our business".