P&O Ferries' Spirit of Britain has passed safety checks and can sail from Dover after more than month in port.
It is one of three ships which have not been able to make the crossing since the company's mass Zoom sacking of 800 staff caused outrage.
The under-fire operator has said it faced a loss of £1 million for every day it couldn't sail, putting its estimated losses at £36m.
It hired low-cost agency staff at well below minimum wage in an effort to stay afloat and justified the move by saying it was legal under Maritime Law.
But whether that saving has been wiped out by the prolonged inactivity is unclear.
Earlier this week, seven agency crew members were sacked for drinking on the job at Dover.
On Friday night, a Maritime and Coastguard Agency spokesman 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 confirmed on Saturday morning that services between Dover and Calais remain suspended.
Pride of Hull (Hull – Rotterdam), Norbay (Liverpool – Dublin) and the European Causeway (Larne – Cairnryan) have also been cleared to sail.
European Highlander sails the Cairnryan to Larne route.
Pride of Kent (Dover – Calais) remains under detention.
Two other ferries are yet to have their full Port State Control inspections.