Arran's long-delayed new ferry MV Glen Sannox will be taken out of service almost straight away for a mandatory overhaul.

The announcement came after it was revealed that sea trials for the vessel had been delayed by two weeks - though the Ferguson Marine shipyard insisted the planned completion date at the end of this month remained unchanged.

It has also been revealed by Transport Secretary Fiona Hyslop that she expected to get the completed business plan for the long-delayed upgrade of Ardrossan Harbour "shortly".

A new plan was called for last year due to rising costs of the projects - but wrangles over who pays have pushed the review back by months, prompting fears that the upgrade could be cancelled.

Due to the deterioration of the harbour, the Glen Sannox is expected to sail from Troon to Brodick when it finally begins service.

The new ferry, which is arriving six years late and four times over the original budget, is expected to operate on the Arran route this December.

But operators CalMac have scheduled the vessel's annual maintenance and re-fit  that month as well.

They decision was made after consultation with the Isle of Arran Ferry Committee.

The two-week refit is required to permit the ferry to operate for a year, and delaying it into 2025 is seen as being more disruptive. The work includes inspection of the underwater section of the hull, which is required by the Maritime and Coastguard Agency.

Glen Sannox will operate from Troon because of delays to the upgrading of Ardrossan harbour to accommodate it.

The ferry’s refit will be completed ahead of the overhaul of the current main vessel on the route, 31-year-old Caledonian Isles because of the scale of work which the ageing ferry requires.

The Caledonian Isles is due to return to service near the end of next week after being out of service since January due to £6.5 million of steelwork repairs.

A CalMac spokesperson told The Scotsman: “Glen Sannox is scheduled to enter annual maintenance on December 3.

“Whilst it may seem less than ideal to bring the vessel into service, then dock it shortly after, December represents the best time of year for her overhaul in the context of the schedule for the wider fleet.

“All of our major vessels enter annual overhaul throughout winter due to service demands across all routes in the summer.

“Glen Sannox will serve Arran, and the other major vessel for this route, Caledonian Isles, is scheduled to be in dock for most of January.

“Then, February and March are the most congested time of year for major vessel overhaul. Service demand begins to increase towards the end of March, when the summer timetable recommences.

“We consulted with Isle of Arran Ferry Committee on the timing of the new vessel's overhaul and December was their preference."

Bill Calderwood, the ferry committee’s secretary, said: “We were offered two possible dates, but as this will be an annual schedule we selected the December option as the long-term best option.

“This year gives an opportunity for any early teething issues to be corrected before we expect Glen Sannox will support our Christmas and New Year demand along with Caledonian Isles.

“We await confirmation of the vessel’s introduction to service, which is urgently needed.”

Meantime, Scottish Government-owned Caledonian Maritime Assets Limited (CMAL), which owns CalMac’s ferries, told The Scotsman that its own sea trials to test Glen Sannox had been postponed from this week to September 16-20.

Chief executive Kevin Hobbs said last month would be the “acid test” of the vessel’s capabilities and involve a 12-hour endurance trial which would run it “exceptionally hard”.

He said: “If anything were to go wrong, that's when it will go wrong, but we're not anticipating that.”

Ferguson Marine said the date had been changed for logistical and operational reasons, and the ferry was still due to be completed on September 30.

Transport Secretary Fiona Hyslop told MSPs: “Whilst it is regrettable that there has been further delay to the delivery, I am pleased to confirm that the arrangements to ensure a smooth handover and entry into service are well underway.

“Reports from the sea trials and work to date have indicated that the vessel is performing well.

“We look forward to her entering service and serving the Arran community. Discussions are already underway with communities around the timetables when she initially enters service from Troon.”

On the Ardrossan harbour upgrade, which is seven years late and has yet to start, she said: “I understand all outstanding elements are being completed and I will receive the new business plan shortly.”