Irish Ferries owner ICG has said that its Ulysses ferry will be out of service for longer than expected.
The ferry, which operates on the Dublin/Holyhead route, reported technical difficulties on 24 June.
The vessel entered dry dock in Belfast on 28 June and ICG had thought that repairs to the vessel were expected to take no longer than five days allowing it to resume service on 4 July.
"We have now been informed by service engineers that the issue is more serious than originally anticipated and they expect the vessel will be out of service for a further period of one to two weeks," ICG said in a statement today.
ICG said that it will adjust the schedules of its other vessels to minimise the disruption to customers as much as possible.
It said the Dublin-Holyhead route will operate with the Epsilon on the Ulysses schedule alongside the Dublin Swift, which will operate additional evening sailings.
Irish Ferries apologised to all its passenger and freight customers for this technical disruption.
"Despite our best efforts we can find no replacement tonnage in what is now the peak tourism season," the company said.
It added that this marked the first major disruption on the Ulysses since the ferry's deployment on the Dublin Holyhead service in 2001.
Irish Ferries was forced to cancel of all summer sailings from Dublin to France on its new WB Yeats ship last month.
Almost 20,000 people who were due to travel on the ship between 30 July and 17 September were affected.