The operator is now looking at redeploying or chartering other vessels to help meet demand because it has no spare large ferries available.
Caledonian Isles is being repaired in Troon after suffering an engine failure and hitting the harbour in Ardrossan on Sunday during the busiest weekend so far this year.
The Isle of Arran, a substitute ferry drafted in to take over the main Ardrossan-Brodick route – one of CalMac’s busiest – can only carry half as many vehicles, which has caused major disruption to travel to and from the island.
CalMac has cancelled all bookings on the route.
This has meant that all but priority drivers, such as those carrying food and fuel supplies and people going to health appointments, are having to queue for sailings.
The engine which failed had undergone routine maintenance fewer than three months ago, the company told The Scotsman.
A spokeswoman said: "Scheduled maintenance was completed on the port [left] main engine in accordance with the original equipment manufacturer’s recommendations during the Caledonian Isles’ annual overhaul period between January 7 and February 3.”
CalMac managing director Robbie Drummond said of the fault: "We appreciate the impact this is having on islanders and visitors alike and we are doing all we can to help passengers complete their journeys.
"We apologise to our customers for the inconvenience caused by this technical breakdown and can assure them that we are doing all we can to minimise the impact.
“The vessel is currently in Troon where repairs are being carried out.
"Engineers are working on a 24-hour basis and expect to complete works to bring the vessel back into service on May 3.
"This will be continually reviewed as repairs progress, and we will keep our customers and communities updated if there is a change to this date.
“Isle of Arran will continue to operate the Caledonian Isles timetable on the Ardrossan-Brodick route.
"As this vessel has less capacity, lifeline traffic such as food deliveries are being prioritised.
"Passengers are being advised that all bookings will be cancelled up until May 3 and that this will be a turn up and go service for vehicles.
"Customers can however book travel as a foot passenger on Isle of Arran [ferry] during this time.
"We have no spare large vessels to use, so we are also exploring vessel redeployment and charter options."
He said a second vessel had been added to the secondary route from Claonaig on the Kintyre peninsula to Lochranza.
But resumption of CalMac’s Ardrossan-Campbeltown service would be delayed from next Thursday until the following week.
Bill Calderwood, secretary of the Arran Ferry Committee, which represents passengers, said: “The news that the damage to the engines is more severe than first anticipated is devastating to every aspect of life on Arran.
“Vehicle traffic is being prioritised in line with a listing for essential services and to ensure lifeline supplies.”
Scottish Conservatives transport spokesperson Graham Simpson said: "This is just adding insult to injury for islanders.
"Sadly, it’s become almost the norm.
"The SNP are entirely to blame for this disgraceful situation.
"If they had done things properly, Arran could have had a new ferry by now.”
His Scottish Labour counterpart Colin Smyth said: “It’s a damning indictment of the chaos that has engulfed our ferry network under the SNP that no one is surprised when this happens.
"Islanders simply wonder when the next cancellations will happen”.
Arran Ferry Action Group chair Sam Bourne said: “The next few weeks until Caledonian Isles returns to service will be a very challenging time for all.”