Pentland Ferries pulled out of a contract it had been expected to sign to operate the MV Pentalina on the main Ardrossan-Brodick route from August.
The extra vessel is desperately needed because Covid restrictions have cut capacity on CalMac’s ferries by around two-thirds.
Extra pressure on routes is likely from more people holidaying in Scotland this summer because most foreign travel without the need to quarantine on return is barred.
Industry sources said Pentland Ferries had been likely to have face calls for staff pay rates to increase to match CalMac’s superior salaries.
There is also understood to have been concern about the Rail, Maritime and Transport union’s hostility to the Pentalina being used by CalMac.
The union expressed concern about “unsafe” internal remodelling of the vessel, but Pentland Ferries said a change to the cafe servery was a “minor alteration” and would be removed on instruction from the Maritime & Coastguard Agency.
It is believed the contract signing had been imminent, with Pentland Ferries to operate the vessel with its own crew, with CalMac providing catering staff.
The catamaran was also earmarked for the popular Oban-Mull route this winter, and back-up on other routes such as Ullapool-Stornoway.
CalMac managing director Robbie Drummond said: “Berthing trials had shown [Pentalina] could successfully operate services to Arran and Mull and we were looking forward to her deployment.
“Unfortunately, Pentland Ferries have decided not to enter into a charter agreement for the use of the MV Pentalina on our network.
“We are surprised and disappointed by this unexpected news.
"However, this was entirely a decision for Pentland Ferries in relation to the use of its vessel.
“Whilst disappointing, we were able to develop an understanding of where the vessel could be used and where there would be limitations as a result of the berthing trials and operational issues with the vessel’s single-ended design.
“I know this news will also come as a great disappointment to our communities, particularly those we had been discussing the detailed options with.
"I thank them for their patience and support as we continue to search for suitable alternatives available for charter to enhance capacity and resilience within our network.”
Pentland Ferries blamed “issues outwith our control” the decision.
Managing director Andrew Banks said: “It has become clear that issues outwith our control are likely to arise over the lifespan of the contract to threaten the commercial viability.
"Unfortunately these issues, which fundamentally stem from the very different status of a public-funded service compared to a small private operator such as ourselves, have given us no alternative but to withdraw from the discussions.
"We cannot commit to a scenario whereby external factors have the potential to cause severe risk and reputational damage to the business that we have worked so hard to build, and unfortunately we have therefore let CalMac know of our decision.”
The Pentalina can normally carry up to 247 passengers and 70 cars, or eight articulated lorries and 26 cars.
The ferry operated since 2009 between Orkney and Caithness but was laid up prior to the berthing trials and had not been in passenger service.