P&O Ferries has resumed cross-Channel sailings for the first time since it sacked nearly 800 seafarers.
The vessel Spirit of Britain departed Dover for Calais shortly after 11pm on Tuesday carrying freight customers.
Passenger services are expected to resume early next week.
Read more: Hundreds of P&O ferry passengers left stranded off NI coast
Earlier in the day another P&O Ferries ship, European Causeway, was adrift five miles off the coast of Northern Ireland for more than an hour on Tuesday afternoon after losing power.
The company was widely condemned after replacing 786 crew members with cheaper agency staff on March 17.
Chief executive Peter Hebblethwaite told a joint hearing of the Commons' business and transport committees later that month that P&O Ferries broke the law by not consulting with trade unions before implementing the decision. He has rejected calls to resign.
Rail, Maritime and Transport (RMT) union general secretary Mick Lynch said: "After yesterday's vessel ran adrift off the coast of Larne, no P&O ferry should set sail on safety grounds.
"Staffing ferries with undertrained, ill-equipped, over-worked and grossly underpaid seafarers blatantly undermines maritime safety.
"There will be more safety-related incidents on the P&O fleet under these intolerable owners and we can only hope that they do not escalate in seriousness.
"Instead of taking that gamble with worker and passenger safety, the Government must step in now and take over the running of all P&O vessels."
On Tuesday, the Trade Unions Congress called for a public and commercial boycott of the firm, claiming it deserves "pariah status" for the way it treated its employees.
P&O Ferries' suspension of Dover-Calais sailings after the sackings led to a shortage of capacity on the key route, sparking long queues of lorries on coastbound roads in Kent in the run-up to Easter.
Spirit Of Britain was detained by the Maritime and Coastguard Agency on April 12 after safety issues were found, but was cleared to sail on Friday.
European Causeway, which can carry up to 410 passengers, got into difficulty on Tuesday after losing power while sailing between Cairnryan, Scotland and Larne, Northern Ireland.
The Marine Traffic website stated the ferry's automatic identification system status had been set to "not under command", which is reserved for use when a vessel is "unable to manoeuvre as required by these rules and is therefore unable to keep out of the way of another vessel".
A spokesman for P&O Ferries said it had been a temporary issue and the European Causeway had travelled to Larne "under its own propulsion".
He added that "all the relevant authorities have been informed" and "a full independent investigation will be undertaken".
Read more: Full P&O service from Northern Ireland to Scotland resumes
Read more: Two ships inspected as P&O ferries tries to resume normal operations
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