P&O Ferries was forced to reverse an attempt to pay its cheaper seafarers less.
It comes after the RMT Union had received reports of agency workers at Dover (England UK) being asked to sign contracts replacing the old ones with reduced payment.
The union reported P&O to the Maritime and Coastguard Agency, which ensured the new seafarers retained the original wages.
P&O Ferries sacked ~800 employees in March and brought in cheaper agency workers on some of its vessels, in a move it said would ensure the future of the ferry business.
However, the National Union of RMT (Rail, Maritime and Transport Workers) said the company was now "trying to bring in an exploitative model, with the lowest possible standards they can get away with".
According to RMT, a seafarer on Spirit of Britain at Dover UK had initially contacted them begging for help in a dispute over pay.
On Friday, the Spirit of Britain ferry was cleared to restart sailing following inspections by the Maritime and Coastguard Agency. The ship had been held at the port since April 12 due to a number of unspecified deficiencies, the regulator said.
It is P&O Ferries' first Dover (England) to Calais (France) ferry to restart operations, with services scheduled to resume Tuesday morning, April 26.