The two ferries being built for CalMac are facing another delay of two months.
The warning from Ferguson Marine CEO David Tydeman said the state-owned shipyard is being hit with supplier issues.
In a letter to MSPs, the head of the Port Glasgow-based firm pointed to delays with the LNG propulsion systems for the vessels.
The LNG system is one of two engines on the ferries.
It represents another setback for the building of the MV Glen Sannox and the unnamed Hull 802.
They are already six years late and more than £250m over budget. The original estimate in 2016 was £97m while the current estimate stands at £360m.
Tydeman said Ferguson Marine is “urgently exploring” what it can do to mitigate the supplier issue and has said he will update parliament by the end of January.
The CEO also told Holyrood’s net zero, energy and transport committee that bad weather had prevented the ship from moving to a dry dock this week.
“The ship was ready as planned to move under her own propulsion,” he said, “and it’s disappointing that we missed this significant milestone to demonstrate the good progress with the programme.
“Due to advance bookings for the dry dock for other vessels, we will now have to take our turn, proceed with the trials programme in January to March, and have booked the earliest available dry dock dates of April 3 and 7.”
The MV Glen Sannox is close to completion and is meant to begin sea trials this January.
Hull 802 meanwhile is set to launch on March 12 next year.
Wellbeing Economy Secretary Neil Gray said the news is “extremely concerning”, adding: “I also note with deep disappointment that the CEO has also signalled a risk of further delay, and I have continued to impress upon the CEO that any further slippage in the programme would have a considerable impact on the island communities.
“I also met with the chair of Ferguson Marine Port Glasgow and have impressed on him the need to ensure that every possible mitigation to further delays is in place.”
Scottish Labour transport spokesman Alex Rowley said: “It beggars belief that even more delays seem to be on the horizon for these long-awaited ferries.
“The Cabinet Secretary’s sympathetic words will ring hollow with the islanders and shipyard workers dealing with years of ferry chaos on the SNP’s watch.
“The Scottish Government must set out a real plan to deliver these ferries without any more delays or cost increases, so island communities can get the service they deserve and Scotland’s shipbuilding industry can thrive.”
Scottish Conservative transport spokesman Graham Simpson said: “These are shocking revelations from Neil Gray and David Tydeman. It looks highly likely that the cost of building the Glen Sannox could rise again and there could be yet another delay.
“It goes from bad to worse and all the while islanders are the ones who are suffering.
“Every time we are given a completion date, it slips. At some point Neil Gray is going to have to say how much more money he is prepared to put into the yard. So far it has been a blank cheque.”
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