The Loch Seaforth underwent major repairs in Greenock after suffering an engine failure in April, the breakdown causing disruption to CalMac's west coast network as other ferries had to be used as cover.
An investigation into the breakdown of the vessel has found piston screws – which should have been replaced in 2019 – failed to work properly which led to debris entering the oil system.
CalMac has confirmed a full and independent report has been commissioned to investigate why the screws were not replaced when scheduled to do so.
While these findings explained some damage, investigations to determine the full extent of the breakdown are ongoing, the report said.
CalMac managing director Robbie Drummond said: “I would like to reiterate my sincere and heartfelt apology for the disruption this incident and the knock-on effects it caused.
“We are taking full accountability for the incident and are already investigating exactly what happened.”
He added: “Technical investigations by the engine manufacturer are ongoing, including metallurgical analysis on the screws to determine the root cause of the failure.
"We will implement all recommendations in full made by the independent investigation company to mitigate against situations like this happening in future.”
Mr Drummond said results will take up to eight weeks and may not be conclusive.
Loch Seaforth was originally due to return to service on 17 May, but further damage caused by debris entering the oil system meant services were delayed further until 31 May.
Following successful sea trials on the ferry last week, the vessel returned to full service
Transport minister Graeme Dey described the ferry’s travel disruption over the last two months as “intolerable” for islanders.
In response to the ferry breakdown, Western Isles SNP MSP Alasdair Allan said he had been “deluged by emails from families desperate to visit relatives on the mainland who have had long-booked tickets cancelled, businesses who can no longer get building materials to the islands or export shellfish, and tourist businesses struggling to cope with the news that all bookings are suspended for a fortnight."
He said the six-year-old Loch Seaforth “should not be experiencing technical issues of this severity”.