The official Isle of Arran Ferry Committee, which represents islanders, said the project to improve Ardrossan harbour must start as soon as possible for the main Arran link to Brodick.
Ferries will temporarily be switched to Troon while the work is underway, which may include the new Glen Sannox vessel, which is due to be completed at the Ferguson Marine shipyard in Port Glasgow by September as the route’s main ship.
Committee secretary Bill Calderwood told The Scotsman the body “reflects the growing frustration from the community to all parties involved with the slow progress of this project”.
He said: “We support the development of Ardrossan as our mainland port, but action is urgently required to ensure that work commences as soon as possible to provide a suitable berth for the Glen Sannox and other vessels needed to support what has previously been the busiest passenger route on the network and provide the services required to sustain the islands’ economy and quality of life for the residents.
“The planned refurbishment work needs to be completed as soon as possible.”
Campaign body Arran Ferry Action Group said it thought an agreement over the division of costs had still to be reached between Transport Scotland and Ardrossan harbour owners Peel Ports Group to enable the work, likely to take at least 18 months, to go ahead.
Chairman Sam Bourne said “very rapid” progress was required if work was to start this autumn.
He said “any further delay will push the return of the service to a renewed Ardrossan in a new vessel past the current target of spring 2024”, which was “now a realistic prospect”.
Mr Bourne said: “This will extend the period the service is diverted to Troon and add to the continuing negative social and economic effects being caused to the Isle of Arran by the ongoing disruption to our ‘lifeline’ ferry service.
"They are still stuck at the start line, still in negotiation.
"Nothing can actually proceed until they have agreement.
"So the timeline just keeps sliding to the right, as it has on at least two or three occasions already.
"On the positive side, the required modification works at Troon are progressing well and are on schedule to complete this summer.”
However, Mr Bourne said sailings would take one third longer and turn-around times would be lengthened by the layout of Troon harbour, which could reduce sailings from five to four a day.
Transport Scotland said it shared islanders’ frustration.
Its spokesperson said: “Detailed discussions continue with Peel Ports Group on project delivery options and related commercial agreements.
"These are challenging discussions, but we share the frustrations of the communities on Arran and are determined to reach a resolution that allows us to progress to the tender stage and ultimately deliver this important project.”
The agency said the date of the service switching to Troon was “still to be finalised”.
It added: “Works are currently well underway at Troon to facilitate the use of that port during that period and to allow its use as the new port of refuge for the Arran service beyond this.”