CALMAC is making use of a ferry user priority list as it emerged its biggest vessel was hit by engine issues leading to cancellation of vital island ferry services.

Services to and from one of Scotland's busiest lifeline ferry routes were suspended on Tuesday and into Wednesday morning after its newest major vessel the MV Loch Seaforth suffered technical issues.

CalMac had earlier said that it was sourcing a spare part for Loch Seaforth which it hoped to have fitted on Tuesday.

An engine failure of MV Loch Seaforth two years ago caused seven weeks of chaos across Scotland's lifeline ferry network.

It is not yet clear how long the vessel is likely to be out of action for.

A coalition of island hauliers has previously said it was taking legal advice in a row over the use of priorisation brought in by the Scottish Government-owned ferry operator in the midst of continuing disruption to lifeline services.

The 'disruption management traffic prioritisation framework' emerged ahead of the summer timetables coming in on March 31 and while at the time three key vessels remain out of action.

Now it has emerged that while six vessels have been requiring repairs since Easter - prioritisation has been brought in on one of Scotland's busiest lifeline routes.

It came as services, including a vital early morning freight journey, were cancelled between Ullapool on the mainland and Stornoway on the Isle of Lewis as the nine-year-old MV Loch Seaforth was hit with issues with its engine control system.

READ MORE: Row erupts over CalMac ferry user 'priority list'

CalMac suspended services for the remainder of the day on Tuesday afternoon and due to a backlog of traffic, users were invited to use the link between Uig on the Isle of Skye and Tarbert on the Isle of Harris. Tarbert is 36 miles away from Stornoway, while Uig is 135 miles away from Ullapool.

The Herald: Loch Seaforth leaves James Watt Dock, Greenock after engine repair at Dales Marine.

CalMac said the service would be prioritising freight, including food, medical supplies, animal feed and "time sensitive loads".

Any remaining space would then be offered to general traffic.

The ferry operator's port teams were to handle the "prioritisation of traffic".

The concerns over prioritisation surround general commercial traffic that block-books on the ferries, which according to the CalMac's priority list document "does not have any priority over that of non-block booked traffic".

Hauliers fear that means that bookings can be removed to be given to other people in the midst of service disruptions.

One ferry user group official said that prioritisation had become "a deplorable symptom of the current state of ferries on the west coast".

"Notwithstanding prioritisation, there will be huge detours that people will have to take to get around the fact there isn't a ferry service, even if the Uig alternative is available. There will be folk who will just decide that it is best just to get a flight if it is available."

The 31-year-old MV Loch Tarbert, 32-year-old MV Loch Fyne were added to the roster of vessels needing repairs over Easter Sunday and Monday causing further headaches for the Scottish Government-owned ferry operator. They have returned to service.

Three more of CalMac's fleet MV Caledonian Isles, MV Hebridean Isles and MV Clansman have remained out of action since the summer timetable began on April 1 having spent weeks on the sidelines for repairs after problems discovered after overhauls.

READ MORE: ScotGov 'won't intervene' over £60m cost to resolve Corran ferry chaos

The issues have caused spin off disruptions across the west coast of Scotland network as ferries are shifted to cover for shortages.

The ferry operators have been forced to suspend mainland services linking Portavadie and Tarbert till the end of Tuesday as the dedicated ferry was re-deployed to support the Largs to Cumbrae service.

The First Minister Humza Yousaf when asked about the issues for the islands with the latest ferry problems said: "Of course, any ferry disruption is extremely regrettable. We know how important particularly those lifeline services are for our island communities. But it's why we invested almost £2bn  in ferries and infrastructure since 2007. We've got a plan of about another £700m investment over the course of the next five years. So we'll continue to invest in that ferry infrastructure. But we know just how important it is and any disruption is undoubtedly regrettable.”

The Herald: MV Alfred, the second catamaran ferry bought by Pentland Ferries, and delivered in 2019. Image credit: Pentland Ferries..

Last month, ministers spent £9m to charter an 'emergency' vessel for nine months in the wake of continuing disruption to lifeline ferry services.

MV Alfred is due to join the fleet from April 18 when she will undertake berthing trials before being made available for service.

The vessel which is at the centre of a crash investigation dating back to the summer of last year, will be chartered from Pentland Ferries who will operate services on behalf of CalMac.

The latest issues come after South Uist islanders had their ferry services restored for Easter in a u-turn, but still face weeks without sea links due to a vessel shortage.

Some 18 of of CalMac's 35 working ferries deployed across Scotland are now over 25 years old.

The oldest in the CalMac fleet is is the Isle of Cumbrae which is 47-years old.

Two new lifeline ferries Glen Sannox and Hull 802 were due online in the first half of 2018 when Ferguson Marine was under the control of tycoon Jim McColl, with one intitially to serve Arran and the other to serve the Skye triangle routes to North Uist and Harris, but they are over five years late. The last estimates suggested the costs of delivery could quadruple from the original £97m cost.

Transport minister Kevin Stewart said: “Unfortunately there have been ongoing technical issues with vessels resulting in delays to the annual overhaul programme and cancellation of sailings. The expertise and responsibility for operational decisions regarding the Clyde and Hebrides Ferry Services lies with CalMac as the operator, but we recognise this issue is not just about transport performance in itself - it’s about delivering the confidence needed to sustain island populations.

“Regrettably communities have been greatly impacted and we fully recognise the need to improve reliability and confidence in services. Ferry operators prioritise food supplies during disruption, and are part of the local resilience partnership, they remain in contact with local communities and hauliers. CalMac are also considering additional sailings on other routes to help manage capacity in light of the ongoing issue with the MV Loch Seaforth."

A CalMac spokeswoman said: “On arrival into Ullapool [Monday night] MV Loch Seaforth reported an issue with its engine control and monitoring system. Engineers are currently investigating the defect and are in contact with specialist contract support and will resolve the issue as soon they possibly can.

“We are very sorry for the disruption and inconvenience that this technical fault is causing to our customers, and we are grateful for their patience."