The Energy Secretary has been asked for more information about “regrettable and concerning” developments relating to much-delayed and over-budget Arran ferries.

The Scottish Parliament's net zero, energy and transport committee has written to Neil Gray seeking clarity after he gave evidence during its meeting on September 12.

The Glen Sannox – the first of two ferries being built at the troubled Scottish Government-owned shipyard Ferguson Marine – was due to be in service by the spring.

However last month the yard’s chief executive David Tydeman said the Maritime and Coastguard Agency (MCA) had “reassessed the application of ‘cargo ship’ rules”, meaning modifications were required to the vessel.

He said this will mean sea trials will move into the first quarter of the new year, and the commissioning of the ship's liquified natural gas (LNG) systems, to take place at Troon, will also move to after Christmas.

The Glen Sannox (hull 801) and Glen Rosa (hull 802) were due to be in service in 2018 at a price of £97 million, but delays have led to spiralling costs which could amount to more than three times that figure.

In the letter to Mr Gray, committee convener Edward Mountain said the update from Ferguson Marine contained “important new information” that the MCA had “reassessed the application of ‘cargo ship’ rules to the crew spaces of passenger ferries (on a broader basis than just Ferguson and Glen Sannox/802)”.

The letter adds: “It also mentions ‘other approval issues’ as arising. These mean sea trials and the commissioning of liquid natural gas facilities are pushed into 2024.

“On September 12, you described these developments as ‘largely, but not exclusively, due to the MCA taking a different approach to regulation of crew escape from 801 (the Glen Sannox)’.

“You said that this was ‘not a minor thing’ but was ‘pretty fundamental to whether the design of the ferry allows it to sail’.

“These developments are obviously regrettable and concerning.”

In the letter, Mr Mountain said he is seeking information on matters including the implications of the rules, or any “different approach” to the rules, for vessels in the Caledonian Maritime Assets Ltd (CMAL) fleet other than 801/802, or for ships under construction.

He also asks for Mr Gray’s understanding of the Glen Sannox completion costs, itemising this and setting out estimates of the additional costs resulting from meeting the MCA’s requirements on crew exits and the “other approval issues”.

The committee has also written to MCA chief executive Virginia McVea seeking clarity on issues such as how long discussions between Ferguson Marine and the MCA on final design sign-off for the ferries have been taking place.

A Scottish Government spokesperson said: “Any further delay to the delivery of these lifeline vessels is extremely disappointing.

“We are continuing to engage with Ferguson Marine to fully understand the issues that have emerged and to explore all possible options to mitigate any additional hold ups.

“We have received the committee’s letter and will respond to its supplementary questions, including on Ferguson Marine’s engagement with the MCA, in due course.”

The MCA has been asked for comment.

Meanwhile, transport minister Fiona Hyslop has told MSPs that a review of the award of a contract to build the two new ferries is expected to be completed "fairly soon".

Speaking atHolyrood's public audit committee on Thursday, transport minister Ms Hyslop was asked when the review will be published.

She replied “That’s a matter for the CMAL board, I think it would be fairly soon.

“I’m not responsible for the timescale for that.”