A total of £35 million has been spent on redesigns of the Arran ferry that is already late and over-budget, the head of Ferguson Marine has admitted.

The Glen Sannox - and its sister vessel the Glen Rosa - have been beset by issues which led to multi-year delays and cost overruns which have put the final price at £360 million, compared with the initial £97m price tag when the contract was signed.

In an appearance before the net zero, energy and transport committee at Holyrood on Tuesday, Ferguson Marine chief executive David Tydeman said almost 10 per cent of the final estimated price tag has been spent redesigning the first of the two ships.

He accepted that "lots of mistakes" and "simple errors" were made during construction.

And he also said squeezing components into the ferries was more challenging than the design of the Royal Navy's latest warships, the new Type 26 frigates.

"I estimate that 50 per cent - about £35m - of the increases over the last 18 months on Glen Sannox have come directly from re-working the design, sometimes regrettably involving two or three iterations," he said.

Asked what design issues had been found on the ferry, the chief executive said work involved "re-routing cables, re-routed pipework" and gave the example of hydraulic pipework used to open the clamshell doors at the front of the vessel was installed with mild steel as opposed to stainless steel.

With design now complete, Mr Tydeman said, there is an opportunity to secure fixed price contracts for the remainder of the build of the Glen Rosa.

"Through this we now have, perhaps for the first time, an opportunity to stay within the budget and contingencies," he said.

"Overall, combining the £120.3m incurred by (Ferguson Marine Engineering Limited) between 2015 and 2019, and the latest forecasts I've set out, Glen Sannox will have cost just over £200 million to complete and Glen Rosa is expected to be £40m less, reflecting the benefit of being the second ship."

In his opinion, the chief executive told the committee, the ships should have cost around £70m each to build - less than half of what he estimates the final cost will be.

Mr Tydeman, who said he was aware within three months of taking over at the nationalised yard in January of last year he was faced with a "complex problem", told MSPs he was more confident about the accuracy of the cost estimates than the timetable for delivery of the vessels.

The Glen Sannox is due to be delivered in March next year - six and a half years after the hull was launched - while the Glen Rosa has been slated for handover at the end of May 2025.

Under questioning from SNP MSP Ash Regan, Mr Tydeman said: "I am more confident in the pricing that I am in the delivery dates, because there's an uncertainty in the trials process.

"Once we start to run the engines, test the ship, over the coming months in January, February particularly, that's when we'll know whether the design works, whether any vibration issues cause things to come loose."

He added: "If the trials go smoothly and the handover happens before the end of March for Glen Sannox, then I'm confident we can stick to the programme with Glen Rosa following from that.

"If we get some delays in the trials from Glen Sannox then it'll have a cascade effect onto Glen Rosa."