The latest setback was blamed on a shortage of skilled workers and Covid restrictions at the Scottish Government-owned shipyard in Port Glasgow.
MV Glen Sannox, being built for the main Arran route between Ardrossan and Brodick, is now due to be finished between July and September next year compared to the previous estimate provided to MSPs last August of between April and June next year – three months behind.
Its unnamed sister ferry, codenamed hull 802, earmarked for the Skye-Harris-North Uist route, is now scheduled for completion between April and July 2023, compared to between December 2022 and February 2023 – up to five months behind.
A spokesperson for Ferguson Marine (Port Glasgow) said: “The Covid-19 pandemic caused six months of disruption in 2020 and productivity has continued to be impacted due to a further shutdown in January 2021 and the introduction of additional safety measures.
"The timeline impact of ongoing disruption has been calculated as seven weeks, with additional costs of £1 million, which reflects the update given to Parliament in March this year.
"It brings the total Covid-19 costs to £4.3m, which is treated as an exceptional item and does not affect the overall project budget.
"The overall project budget remains stable and unchanged at £110.3m.
“Recruitment challenges since late 2020 have caused a delay of eight weeks as the shortage of local skilled labour meant that Ferguson’s had to meet resource requirements by subcontracting smaller fabrications to Scottish businesses, which has supported 25 jobs, and introducing overseas workers.
“The report also outlines achievements and progress to date, including a major milestone in the build of MV Glen Sannox with the completion of structural work.
"Progress is visible with the installation of a reworked funnel and newly-constructed mast, as well as completion of the structure around the stern and inside the hull.
"Remedial work has been completed on hull paintwork and the first layers of protective paint have been applied to the aluminum superstructure.
"Completion of the structure makes way for outfitting of the vessel, which includes the installation of 10km of pipework and extensive equipment, plus the creation of public spaces and cabins and full furnishing.”
Tim Hair, turnaround director at the yard, which was taken over by ministers in 2019, said: “I know the further delay to the project will be a disappointment to island communities and others who await the arrival of the new ferries.
"There remains a lot of work to do on the vessels, but it is important to recognise the level of progress too, as well as the significant operational improvements we have implemented to introduce robust and effective business processes.
"We have, in effect, created a functioning shipyard business from a standing start.
“The past year has been extremely challenging.
"We’ve been working under the restrictions and pressure of a global pandemic, and recruitment has proved difficult, with the pool of skilled workers insufficient to meet our resource requirements.
“However, we now enter a new phase of production.
"The milestone on MV Glen Sannox is highly significant because, for the first time in this project, we have a complete vessel structure to work with.”