The Pride of Kent, a P&O ferry, has failed an inspection by the Maritime and Coastguard Agency (MCA) for the third time.
The ship has been detained at the Port of Dover since 28 March and failed a second safety test on 13 April.
An MCA spokesperson said: "There are no further inspections of P&O Ferries at the moment but we will inspect when requested by the company."
P&O Ferries used to have four ferries operating on the Dover-Calais route - but only one has been cleared to resume passenger services since the company sacked almost 800 seafarers in March to replace them with cheaper agency workers.
Drive-on passengers and tourists were able to use the service for the first time in almost six weeks on Tuesday by boarding the Spirit of Britain.
The ship has been crossing the English Channel for the last several days, carrying freight-only traffic.
The ferry operator said it is operating on a reduced timetable on Tuesday and is set to resume a normal schedule on Wednesday.
P&O Ferries suspended its services in March when it announced the redundancies of its staff, which had been made without consultation.
The ferry operator was then told each vessel in its fleet must be deemed safe to operate because of the widespread change in crew.
Upon inspection, the MCA found a "number of deficiencies" on the Pride of Kent and the Spirit of Britain.
The Spirit of Britain was found to have 23 safety failings on 11 April but was cleared to sail again 11 days later.
The other two ships, the Pride of Canterbury and the Spirit of France, are also unavailable because they have not been fully examined.
P&O's passenger services have already resumed on three other UK routes, which are between Liverpool and Dublin, Ireland; between Cairnryan, Scotland and Larne, Northern Ireland; and between Hull and Rotterdam, the Netherlands.