Two ferries due in service in 2018 have been delayed again, with an increase in costs of £8.7 million, Finance Secretary Kate Forbes has said.

The Glen Sannox and hull 802 are being built at the Government-owned Ferguson Marine shipyard in Port Glasgow, but cabling issues will now mean that neither will be in service until at least next year – five years later than planned.

The Glen Sannox will be delayed by eight months and is due to be completed between March and May next year, while 802 will be completed between October and December next year.

Forbes also announced an increase of £8.7 million in projected costs, meaning the overrun has risen to between £122.5m and £126.5m.

The two ferries will now be delivered five years later than initially plannedForbes made the announcement on Tuesday

Some £825,000 directly relates to the cabling, the Finance Secretary said, with the rest covering overheads, staffing and materials.

“I reiterate today that there are no ifs, there are no buts, those vessels must be completed and they must be completed as quickly and as effectively as possible,” she told MSPs.

Forbes added: “I won’t rehearse my frustration, and I know the chamber’s frustration at this updated timetable.

“The Ferguson Marine board and chief executive are aware of the depth of my dissatisfaction with the emergence of the cabling issue and the knock on impact to the timetable.

“I have made it very clear that these vessels must be delivered in line with this schedule.”

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Scottish Tory MSP, Graham Simpson, pushed the Finance Secretary to hold a public inquiry on the ferries, a request which was denied, with Forbes saying there had been two “thorough” investigations, including one by Audit Scotland this week and a committee inquiry held in the last session of parliament.

The announcement comes as a report from Audit Scotland found a “multitude of failures” had contributed to the delays and overspends – with the estimated cost before Forbes’ statement set to be at least £240m.

Initially, both ferries were due to cost just £97m.

Auditor general, Stephen Boyle, said: “The failure to deliver these two ferries, on time and on budget, exposes a multitude of failings.

“A lack of transparent decision-making, a lack of project oversight and no clear understanding of what significant sums of public money have achieved.

“And, crucially, communities still don’t have the lifeline ferries they were promised years ago.”

On Wednesday, MSPs rejected calls for a public inquiry into the delays and overspends.

A Tory motion calling for the inquiry was amended by a Scottish Government amendment by 62 votes to 50. The amended motion was passed by 79 votes to 32.

A Labour motion, which called on First Minister Nicola Sturgeon to take control of shipyard Ferguson Marine, also fell by 50 votes to 62.