The Maritime and Coastguard Agency (MCA) ban on the "Spirit of Britain" sailing raises the prospect of further travel chaos for many over the Easter weekend, after scenes of lengthy delays at Dover in recent weeks.
"The Spirit of Britain has been detained due to surveyors identifying a number of deficiencies which were grounds for detention," an MCA spokesman said.
"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 spokesman did not elaborate on the issues.
P&O Ferries suspended its services on the popular cross-Channel route after its abrupt sacking of nearly 800 workers on March 17.
The company had previously said it could resume the England-France service on Good Friday at the earliest.
The sackings triggered outrage from unions, with the company immediately hiring foreign agency workers to replace them on less than the minimum wage.
P&O is facing a criminal and civil probe after admitting it deliberately flouted their obligations to consult on the job losses.
Its Pride of Kent vessel has already been detained, leading to a shortage of capacity on the Dover-Calais route.
This Easter is the first holiday period since the government lifted all Covid restrictions in England, including on outbound passengers.
