Watch our video report from Abigail Bracken
P&O services between Dover and Calais are suspended today.
The company says have secured space with DFDS for passengers wanting to travel.
A union leader says safety concerns still remain despite a P&O ferry detained at Dover being cleared to sail again.
The Spirit of Britain had been detained at Dover after an inspection found "a number of deficiencies".
It has now been released and is allowed to commence operations.
A spokesperson for the Maritime and Coastguard Agency said: "The Spirit of Britain has been released from detention and can commence operations when P&O Ferries are ready.
"The inspection of the European Highlander is now complete; there are a small number of deficiencies and the MCA requires confirmation that these have been rectified before the vessel is free to resume service.
"There are no further inspections of P&O Ferries at the moment.”
P&O Ferries told customers on Saturday that its services remained suspended but that it had secured space with DFDS.
The ship was detained earlier this month and operator, P&O Ferries was asked to fix the issues raised by the inspection, before inviting the MCA back for a second assessment.
Darren Procter, National Secretary of the RMT said he was not surprised that the ferry has been released because of the time the crew on board have been given.
He said: "They have not been sailing the vessels and they have been doing drills, three, four, five times a day whatever it may be.
"But the problem is going to come later down the line when they change the crew on board the vessel which is going to take place in the next four weeks.
"Whilst they may have passed their MCA inspection at this point the safety concerns still very much remain."
P&O Ferries sacked 800 of its staff on 17 March.