The MCA (Maritime and Coastguard Agency) said inspectors had found a "number of deficiencies".
P&O Ferries said it looked forward to welcoming clients again as soon as mandatory safety tests had been passed.
The MCA said they had advised P&O to invite them back once they had addressed the issues.
P&O Ferries' Dover-Calais routes are now suspended. The company said it would not restart services on the route before Friday at the earliest.
The Maritime and Coastguard Agency has been making its way through inspections of 8 vessels of P&O Ferries to ensure they are fit to sail. European Causeway and Pride of Hull have been inspected and cleared to sail, while Pride of Kent, which normally sails on the Dover-Calais route, remained under detention with Spirit of Britain.