MV Coruisk, which has suffered an engine problem throughout summer, has now been taken out of service from the Oban-Craignure route.
CalMac has said an investigation is underway and they are diverting traffic to other services as well as deploying an additional service.
It is not known how long it will take to fix, but the Mull & Iona Ferry Committee said it is ‘likely to be a matter of days, not hours’.
All passengers booked for the Coruisk are being advised to travel via Lochaline, if space is not available on the Isle of Mull which is not available on most sailings.
The small reserve vessel MV Loch Linnhe is also being brought up to Lochaline to add capacity.
Robert Morrison, operations director for CalMac said: “A problem with the MV Coruisk’s engine propulsion management system has occurred and we have had to withdraw her from the Oban-Craignure route while investigations take place.
“We will work very hard to accommodate some of the displaced traffic as directly as possible on MV Isle of Mull Oban – Craignure service, but inevitably, with the current high demand on all sailings, some traffic will have to route overland via Corran Ferry and to Mull through Lochaline to Fishnish. To support the regular vessel on the shorter crossing, we are deploying a second vessel from the Clyde area which will be on service Thursday 15th July.
“We are conscious the continuing service issue with MV Coruisk is having a direct impact and alternative routing may be difficult for some of our customers.
"Our teams with the supporting external specialists are working strenuously to find a solution to return the vessel to service.”
A spokesperson from the Mull & Iona Ferry Committee which represents the users of lifeline ferry services said: “This is another example of how the CalMac fleet is overstretched, over-age and we are paying the price.
“In addition, had the Pentalina entered service (rather than being deliberately obstructed by the trades unions) we would not be in this situation.
“This will cause inconvenience and financial harm to many of us – but the CalMac staff will be feeling it too.
“Frontline CalMac staff will have lots of unhappy customers to deal with, but they will no doubt do it with their usual helpfulness, consideration and calm.
The committee said the problem is for the Scottish Government to resolve and said they will be writing to them to make that plain.
A Transport Scotland spokesperson said: "Whilst the management and maintenance of the vessels is an operational issue for Calmac, we recognise communities’ frustration at the disruption and the impact it is having.
"The Minister for Transport is given regular updates on the performance of the Clyde and Hebrides network, including the current challenges faced by users of the Mull-Iona ferry service.
“Discussions are ongoing to enable the affected vessel to return to service as soon as possible. Meanwhile, Calmac has deployed the MV Loch Linhhe to Lochaline to provide additional capacity to Mull.”