The first P&O ferry has crossed the English Channel since the company sparked outrage by sacking 800 workers without notice last month.
The firm’s Spirit of Britain departed Dover for Calais just after 23:00 BST on Tuesday, traffic websites showed.
The operator confirmed that the Sprit of Britain will initially carry only freight and no tourist traffic.
It came after another P&O ship spent hours adrift without power in the Irish Sea.
P&O said the incident was caused by a “temporary mechanical issue” that had been resolved.
The ship – which will now be visited by maritime inspectors- returned to port “under its own propulsion, with local tugs on standby,” a spokesperson added.
On 17 March, P&O announced it was sacking 800 of its staff and replacing them with agency workers paid below the UK minimum wage as part of cost cutting measures.
The Sprit of Britain had been held in port by the Maritime and Coastguard Agency since 12 April due to a number of unspecified deficiencies, but was cleared to sail again on Friday.
It is the only P&O ship back in service on the Dover to Calais route, with three others still waiting to pass inspections.
Transport Secretary Grant Shapps has pledged to change the law to force all ferry operators using UK ports to pay staff the minimum wage.
