The MV Hebridean Isles has been taken off the network since February due to a fault with its control system.
It had been scheduled to return to service in June, but will now be delayed until September at the earliest, after repair work could not be completed.
MV Hebridean Isles was initially expected to return to serve the Islay route. The 38-year-old ferry, one of the oldest vessel in CalMac’s fleet, has now been out of action for months.
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The five-week delay will further stretch ferry services across CalMac’s network, which are already at breaking point, with the creaking fleet struggling to cope.
It means the MV Isle of Arran will remain on the Islay route alongside the MV Finlaggan.
Ardrossan-Brodick will continue to be served by the MV Caledonian Isles and MV Alfred.
The Campbeltown ferry service will now be cancelled until the 24th of August.
Robert Morrison, operations director for CalMac, said that his team was working to minimise disruption.
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“We are not currently in a position where the Hebridean Isles can return to service,” he said.
“This is very disappointing news, and I apologise to our customers and communities.
“We have started a detailed review of options for further investigation and maintenance, and the outcome of this review will be discussed internally as a matter of priority.”