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P&O Ferries passengers may have to wait until Thursday 14 April to travel on its Larne-Cairnryan service.
In a series of tweets, P&O said that it expected the route to resume next week.
The European Causeway had been detained in Larne by the Maritime and Coastguard Agency (MCA) over safety concerns, after P&O sacked 800 of its workers.
On Saturday, it sailed, with limited services, on a return journey between Larne and Cairnryan. P&O said "full services" would restart on Sunday.
But it is understood that only freight has been transported on the vessel.
P&O said that the MCA had "completed its most recent inspection of the European Causeway, and have confirmed it is safe to sail".
It said it was "delighted that we will be able to restart full services between Larne and Cairnryan from 8am, Sunday 10th April".
"P&O is pleased to play its part in supporting customers to move vital goods between the Northern Ireland and Scotland," it added.
On Sunday morning, P&O tweeted that the Larne-Cairnryan route was "cancelled until 12/04 for tourist travel but should resume on 13/04".
However it later extended this to Thursday 14 April.
The detention came after the company sacked 800 workers across the UK without notice.
The company recruited new staff, but the MCA detained the European Causeway saying it was concerned about "failures on crew familiarisation, vessel documentation and crew training".
Another P&O vessel, the European Highlander, remained docked in Larne on Saturday awaiting a full Port State Control Inspection.
The ferry operator has been strongly criticised by politicians and unions following the sacking of workers.
However, P&O says its business "would not be viable" without changes to staffing.
P&O Ferries has been contacted for comment.