Delivery of the long-delayed Glen Sannox ferry has been pushed back once again, MSPs have been told.

The Ferguson Marine shipyard said it had been forced to postpone a key test of its liquefied natural gas (LNG) fuel system.

The state-owned shipyard found the process of cooling the pipes to the sub-zero temperatures necessary to load the fuel was taking longer than anticipated.

This meant they would no longer be able to meet the deadline for the handover to the ferry-owning agency CMAL by 30 September. The latest delay is expected to be approximately two weeks.

The ship and its sister vessel, Glen Rosa, were originally due for delivery in 2018 but have faced repeated design and construction challenges.

Both ferries were designed from the outset to be "dual fuel" ferries, able to run on both traditional marine gas oil and LNG.

But problems installing the LNG system have been behind some of the repeated delays in delivering the two ferries, with the Port Glasgow shipyard dealing with these issues over the past year.

Ferguson Marine's interim chief executive John Petticrew wrote to MSPs to inform them of the latest delay.

He said: "The past week has seen us dealing with several technical challenges relating to the cooling of the LNG system, which has delayed the loading, final commissioning of the system and the harbour and sea trials.

"Regrettably, this means that some of the work required after these trials will not be completed in time for the proposed handover at the end of September 2024."

Mr Petticrew added: "I am personally very disappointed to be delivering this news, but I can assure (you) that the team at the yard are doing everything possible to keep as closely as possible to the proposed schedule."

LNG needs to be kept at below -160C in order to remain in a liquid state.

Following the harbour trials, the plan is to put the vessel through its paces at sea, stopping and starting several times and carrying out endurance tests.

Current cost estimates for Glen Sannox are between £145.5m and £149.1m and it is hoped the cost of Glen Rosa will be kept below £150m.

Now six years late, the total bill will be three times the original £97m price tag.

Glen Sannox undertook sea trials earlier this year, but in those tests the 102-metre vessel was not using LNG.

The ferry, which has capacity for 852 passengers, is designed to serve the Arran route with CalMac.

New CalMac ferry Glen Sannox has begun some of its final sea trials before being handed over by the Ferguson shipyard.

The long-delayed ship is being put through its paces using liquefied natural gas (LNG) to power its engines for the first time.

Despite a last-minute hitch that forced a further two-week delay, the ship was last week loaded with the fuel, which needs to be kept at minus 162C.

The ship, currently scheduled for delivery by mid-October, sounded its horn as it passed the CalMac headquarters in Gourock.

The ship underwent its first successful sea trials earlier this year - but only running on conventional fuel because the LNG systems had yet to be commissioned.

Ferguson Marine said the latest tests had got off to a good start, with the ship accelerating to 12 knots as it passed the Isle of Cumbrae.

Over the coming days the trials will evaluate manoeuvrability, acceleration and whether it can reach its contracted speed of 16.5 knots.

These will be followed by "owner's acceptance trials" with ferry procurement body CMAL, beginning on 30 September, and several weeks of crew familiarisation trials by operator CalMac before it carries its first passengers.

The ship was originally due for delivery in 2018, but has been beset by a series of problems including design difficulties and an acrimonious contractual dispute between CMAL and the shipyard's former owners.

Glen Sannox and its sister ship Glen Rosa are the first ferries ever built in the UK capable of running on LNG as well as conventional marine gas oil.

But the technology has added a lot of extra complexity for the Port Glasgow shipyard, and many of the delays have been linked to the LNG systems.

One former Ferguson Marine boss described the challenges of packing the equipment into the tight spaces as more complex than designing a Navy warship.

The most recent delay was caused by difficulties achieving low enough temperatures for the fuel to be loaded at Greenock's Inchgreen Marine Park, where Glen Sannox has been moored.

Earlier this week, the shipyard announced that after four-and-a-half days of cooling it had successfully completed the refuelling operation.

Andrew Milligan, head of engineering at Ferguson Marine, said: "The successful LNG bunkering is a highly significant milestone as it’s the first time it has ever been done in the UK.

"It was a complicated process which required significant safety protocols to be put in place.

"We experienced some unexpected challenges around this process and, unfortunately it has had a knock-on effect to the handover schedule."

The main benefit of LNG fuel is that it is seen as cleaner burning, releasing fewer pollutants such as nitrous and sulphur oxides.

The engines also emit up to 25% less carbon dioxide when running on LNG although some have questioned the green credentials of the technology.

There is a risk of "methane slip" - the release of unburnt gas during certain operations - and methane is itself a particularly harmful greenhouse gas.

The LNG fuel will also have to be imported from Qatar and driven to Scotland from Kent in south east England using diesel road tankers once or twice a week.

The plan is eventually to have dedicated fast LNG refuelling facilities built at the ship's main harbour at Ardrossan, but plans to redevelop the port have stalled.

Glen Sannox will initially sail to Arran from Troon instead, and be refuelled directly from road tankers.

Sister ship Glen Rosa is due for delivery next year - and four slightly smaller ferries using more conventional propulsion systems are being built in Turkey.

A CalMac ferry due to return to active service in the west of Scotland has been hit by a further delay.

The ferry operator said routine checks on the MV Caledonian Isles following berthing trials on 22 September had discovered an "unexpected problem" with the vessel's port gearbox.

The vessel sails between Ardrossan and Arran but has been out of service since February, when an annual service at a shipyard on the Mersey revealed a number of repairs were needed.

Sailings for part of Tuesday and Wednesday cancelled as a result.

Calmac said investigations had begun "immediately after the problem was identified" and were continuing.

A statement said: "We know this unexpected further problem will be concerning and frustrating - our teams continue to work hard to return MV Caledonian Isles to service as soon as possible."

Initially the ferry had expected to be out of service from February until June.

According to the latest update, external, MV Isle of Arran will continue operating the timetabled Ardrossan - Brodick service up to and including the 13.55 Brodick to Ardrossan, North Ayrshire, sailing on Tuesday, but sailings for the remainder of Tuesday and Wednesday will be cancelled.

Calmac conceded some bookings up to this point would be affected, however, and said teams would be contacting passengers whose trips have been affected.

Jamie Greene, the Scottish Conservative MSP for West Scotland, called the news a "bitter blow" for people living on Arran,

He said: "Just days after they hoped this service might finally be back up and running again, it has been hit with more technical problems.

"Sadly, they will not be surprised given that Arran has long had to try and rely on some of the oldest ferries in CalMac's fleet."

A Transport Scotland spokesperson said: "We recognise the impact that delays and disruption have regrettably had on our island communities - including Arran - and are committed to investing in our ferry services.

"Delivering six new major vessels to serve Scotland's ferry network by 2026 is a priority for this government."