Delivery of the long-delayed Glen Sannox ferry has been pushed back once again, after the Ferguson Marine shipyard was 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 handover to the ferry-owning agency CMAL by September 30 as planned.

Both Glen Sannox and sister vessel Glen Rosa were designed from the outset to be “dual fuel” ferries, able to run on both traditional marine gas oil and LNG.

READ MORE: CalMac: First sea trials of Glen Sannox 'overwhelmingly positive'

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

Ferguson Marine’s interim chief executive John Petticrew wrote to MSPs on Thursday.

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.”

The vessel handover has now been delayed by approximately two weeks, he said.

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.

David Tydeman, who was chief executive of Ferguson Marine until March, told the The Scotsman earlier that loading LNG was a “complicated” process involving purging the pipelines of any moisture.

He said: “The time needed to do this was highlighted by the team to me 12 months ago.”

Conservative transport spokesman Graham Simpson said: “This latest delay to Glen Sannox is yet another blow for island communities betrayed by the SNP.

“The LNG system has been at the root of many of the issues with the ferry. As former Ferguson chief executive David Tydeman said, these have been known about for some time.

“It adds insult to injury that no SNP minister has yet been sacked over this appalling scandal which they have presided over for years.”

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

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