Port Glasgow shipbuilders Ferguson Marine Engineering Limited (Fmel) announced it planned to make a claim against Scottish Government-owned Caledonian Maritime Assets Limited (Cmal) - which owns CalMac's ferries - over the Â£97 million contract for the Glen Sannox and a sister vessel.
It said a claim would be lodged "within weeks" over the project to build the ferries which will be able to run on liquid natural gas or diesel.
Fmel has blamed the changes and the newness of the technology involved for delays to the Glen Sannox, which could now enter service on the main Arran route a year late.
It may not now carry passengers until next autumn.
A Fmel spokesman said: "Due to the repeated unwillingness of Cmal to engage with Fmel over the additional costs identified due to unforeseen complexities in building the two prototype, first in class, dual fuel liquid natural gas vessels, we are in discussions with professional claims experts and aim to submit a formal claim to Cmal within the next few weeks.”
A Cmal spokesperson said: “Fmel is under contract to supply the design and build of the two dual fuel ferries.
"It is an industry standard design-and-build contract with a fixed price and defined delivery dates.
"Fmel entered into the contract with full and prior knowledge of the specification and terms of the contract.
"Cmal fundamentally disagrees with any assertion there have been significant design changes to the vessels.
"Minor changes have followed the contractual process and costs for these have been agreed with Fmel.
"These costs have been covered by a 3 per cent project contingency budget held by Cmal.”