Work on two ferries being built at the nationalised Ferguson shipyard will continue for at least a further six months and costs are likely to rise again, Scotland’s deputy first minister has admitted.

The MV Glen Sannox was due to be delivered to Calmac, the west coast ferry operator, by the end of May for service on the Ardrossan to Brodick route.

But John Swinney told MSPs that it would not be ready until the autumn at the earliest because of “persistent design gaps and build errors”.

The other ferry, Hull 802, was scheduled for service in March next year, with the possibility that it would be used on the same route from the mainland to Arran. It is now unlikely to be available until the end of 2024.

Delays have best the vessels from the outset with both originally due to enter service in 2018. The £97 million contract awarded in 2015 has ballooned to £300m.

In parliament yesterday, Mr Swinney apologised to island communities affected by the latest delay.

He said: “It is a matter of great disappointment that a further revision to the timescale for delivery has been necessary. I’m conscious that delays to the delivery of any project can lead to an overall increase in costs.

“That is why the Scottish government will work with both Ferguson Marine and our technical advisers, [Caledonian Maritime Assets Limited], to assess any financial impact to the delivery of both vessels.”

Graham Simpson, from the Scottish Conservatives, said: “The MV Glen Sannox and Hull 802 could each be delayed by up to seven months and we still don’t know what the final cost is. More delays, more costs and islanders left in the lurch. It is a disgrace.”

Neil Bibby, from Scottish Labour, said that the situation was a “scandal manufactured by SNP ministers and they all have their fingerprints on it…Humza Yousaf, John Swinney, Kate Forbes and Nicola Sturgeon to name a few.”

Scottish Liberal Democrat economy spokesperson Willie Rennie said: “John Swinney is a master at defending the indefensible, but even today he cannot defend this set of circumstances.

“He’s got no idea about the final costs, no idea about the final delivery dates and apparently no idea that these bonuses were being paid, even though his government owns the yard.  

“So, what guarantee can he give that this will be the last statement of its kind about these ferries?”  

Speaking after the statement, he added: “I cringed when I heard SNP backbenchers applauding this debacle. If this is success worthy of praise, I’d hate to see failure.”

A report earlier this week from Audit Scotland, the public spending watchdog, raised concerns about the delays and the payment of bonuses to management without government ministers being informed.