The dispute is likely to further inflame controversy over owner Pentland Ferries pulling out of an expected deal with CalMac to operate the vessel to provide much-needed extra capacity on one of CalMac’s busiest routes.
However, the spat may muddy the waters because The Scotsman understands talks foundered principally because Pentland Ferries pays its crew – who would have operated the vessel as part of the deal – less than CalMac.
It was due to provide extra sailings to Arran on the Ardrossan-Brodick route from August, and between Oban and Mull this winter.
The RMT claimed unauthorised changes to Pentalina were made which threatened safety.
However, passengers accused the union of sensationalism.
The Maritime & Coastguard Agency (MCA) has ordered that alteration – made in 2015 – to be removed.
It told the union: “Discrepancies were uncovered with respect to structural fire protection in way of the passenger accommodation and the corridor providing access to the crew accommodation and galley servery”.
The catamaran ferry was replaced on the Caithness-Orkney route in 2019 and has been laid up since, other than for short-term charters, such as cable-laying work, and covering for its MV Alfred successor during routine inspections.
RMT general secretary Mick Lynch said: "We are proud to have stopped this unsafe vessel from coming onto lifeline routes and to have upheld safety standards for CalMac crew and passengers.
“The fact is that the Pentalina has been operating for over five years with a serious safety defect in the fire separation between the galley kitchen and the passenger lounge.
"This would never have happened in a trade union organised workplace, but it is no surprise to this union."
However, the Mull and Iona Ferry Committee said: “As we understand it, those alterations amount to the removal of a fridge from the on-board servery, which consequently had an unintended impact on the fire separation between the galley and the passenger lounge.
"As we further understand from sources close to the matter, the problem can be fixed in a matter of hours.
“It is clear that by sensationalising this ‘scandal’ very publicly and defaming Pentland Ferries and Andrew Banks, the RMT have set out to intimidate the company and prevent the Pentalina from coming into the fleet.”
A spokesperson for Pentland said: “The MV Pentalina has held a MCA-approved passenger ship safety certificate and has been Lloyds Register-classed since going into service in 2009.
"The vessel has undergone annual surveys and inspections by both authorities throughout this time.”
An MCA spokesperson said: "A surveyor from the MCA carried out an inspection on the vessel in question on June 16.
“Following the inspection, the vessel will remain out of commercial service until the identified areas of non-compliance have been addressed.”