The delivery date for MV Glen Rosa, the second of two dual-fuel CalMac ferries being built by the nationalised Ferguson shipyard, has been put back by up to nine months.

The cost of the ship has also risen – by up to £35m – meaning the two ships will cost upwards of £460m, more than four-and-a-half times the original contract price.

Ferguson Marine said it now expected Glen Rosa to be “substantially complete” in the first quarter of 2026, with final delivery taking place between May and June.

The ship, which is to serve CalMac’s Arran route, was originally due for delivery in July 2018.

MV Glen Rosa was launched from the Ferguson slipway in Port Glasgow a year ago in a far more finished state than its identical sister ship MV Glen Sannox seven years earlier.

While there was much “fitting out” work still to be done, including specialist cryogenic pipework for its liquefied natural gas (LNG) engines, it was hoped it could be delivered by the end of September this year.

Ferguson Marine’s newly-appointed permanent chief executive Graeme Thomson apologised “unreservedly” to islanders as he confirmed this was no longer possible.

He said: “This is not the announcement we wanted to be making at this stage and cannot overstate our understanding of the importance of providing realistic handover schedules to support CalMac to provide a more reliable and robust service to the communities it serves.

“No one wants to see the swift delivery of MV Glen Rosa more than Ferguson Marine and we are committed to working hard to ensure the vessel is delivered within this window.”

The update gave no detailed reasons but BBC Scotland News understands there have been knock-on impacts from delays in the construction of the previous ship.

Glen Sannox was finally delivered to its owner, ferries procurement body CMAL, last November after a number of late stage complications with the specialist LNG pipework and commissioning of the engine systems.

Sources at the shipyard said significant resources had to be diverted from Glen Rosa in order get Glen Sannox into service.

In an update letter to MSPs, Mr Thomson said the cost to complete the vessel would rise from £150m to £172.5 plus a further £12.5m for contingencies – taking the forecasted cost to £185m.

Glen Sannox was completed for about £150m but these figure do not include £83m paid out prior to nationalisation, or £45m of government loans that were subsequently written down.

Deputy First Minister Kate Forbes said she had communicated her “disappointment and frustration” at the announcement to the Ferguson board.

“It is imperative that that there is no further delay or additional cost to deliver the vessel beyond this point,” she said.

“Ferguson Marine’s leadership must take immediate and sustained action to restore trust, enforce delivery discipline, and bring this project under control.”

Scottish Conservative transport spokeswoman Sue Webber said the government should take responsibility, and that islanders had been “betrayed at every turn by the SNP’s incompetence”.

She said: “The enormous increase in both the delay and the cost of delivering the Glen Rosa beggars belief – and yet it is totally in keeping with the nationalists’ scandalous mismanagement of our ferry network.”

For the Scottish Liberal Democrats, West of Scotland MSP Jamie Greene demanded an urgent statement to parliament.

“The hard-working staff at Ferguson Marine have been let down by bosses, all paid huge sums of public cash, yet islanders are still waiting on their ferries. Not a single SNP minister has lost their job over this fiasco,” he said.

The £97m contract for Glen Sannox and Glen Rosa, the first LNG-powered ferries ever built by a UK shipyard, was awarded to Ferguson Marine in 2015, and both ships originally were due for delivery in 2018.

But design challenges and disputes between the shipyard’s former owners and government-owned ferries body CMAL led to a stand-off over claims for extra costs.

The shipyard’s management maintained that a poorly-developed specification and repeated interference by CMAL had led to unforeseen complications.

CMAL denied this and claimed the yard’s new owners had underestimated the complexity of the project.

The stalemate eventually resulted in Ferguson’s going into administration and being taken into state ownership in 2019 with the project already about £100m overbudget.

Problems continued after nationalisation with further delays and costs further.

MV Glen Sannox is now in service on CalMac’s Arran route, but the state-owned ferry operator remains under pressure as it awaits the arrival of new replacement vessels for its ageing fleet.

Four other large ferries under construction in Turkey are also delayed with the first of them, MV Isle of Islay, due for delivery later this summer.