THE integrity of a report on the future of the Gourock-Dunoon ferry service has been called into question after Scotland's auditor general used the boss of a rival operator as an 'advisor'.

Gordon Ross, managing director of Western Ferries, was privy to the 'findings and conclusions' of an investigation published prior to transport chiefs deciding to persist with unreliable foot passenger-only boats on the route.

The Audit Scotland report also quoted inflated figures of up to £30m each for two new vehicle-carrying ships.

However the body in charge of the country's ferry fleet has subsequently estimated the cost at £10m per vessel.

Campaigners fighting for the return of car ferries to replace the current small boats — which have suffered more than 4,500 cancelled sailings since 2016 — suggest 'a clear conflict of interest' in Mr Ross having been an advisor to the report.

Auditor General, Caroline Gardner, also took advice from Graham Laidlaw, the head of the ferry unit at Transport Scotland — the government body intent on keeping the passenger-only service in place.

In a strongly worded letter to Ms Gardner, Susanna Rice, convener of the Dunoon-Gourock Ferry Action Group, states: "It is normal for members of advisory groups to get advance sight of reports to offer comment on any mistakes that they find.

"The MD of Western Ferries will have known that the figure of £25-30 million for new vessels for the route was, to say the least, excessive.

"We suggest that that there was a clear conflict of interest for such a person to be an advisor for your report?"

On the use of a senior Transport Scotland official in an advisory capacity, Mrs Rice told Ms Gardner: "This inevitably raises the question of whether your investigation has been compromised.

"In democratic states, audit methodology does not normally permit representatives of the body being investigated to also be its advisors."

Transport Scotland declined to comment on its advisory role to the report, which is titled 'Transport Scotland's Ferry Services'.

A spokesman said: "Unfortunately we don't have any comment to offer on this issue.

"This is for Audit Scotland and the ferry action group."

Former Caledonian MacBrayne boss Martin Dorchester gave the £25m-£30m figure to a Holyrood committee in 2017 but it has been dismissed by campaigners as a 'guesstimate'.

A £50,000 expert study commissioned by the Scottish Government in 2013 found that ships of at least 40 metres in length were needed to cope with changing weather conditions on the route and would cost £6m for each vehicle-carrying vessel.

Caledonian Maritime Assets Ltd (CMAL) says the current 'ballpark' figure for ferries capable of carrying 220 passengers and 40 cars is £10m each.

An Audit Scotland spokeswoman said: "The role and membership of the advisory group are set out in the report. Advisory groups are a routine part of our performance audit work, and members are selected for their individual knowledge and technical expertise on the public service that we are auditing.

"Advisory groups enable us to test our findings and conclusions with the depth of insight required to make sound judgements, but the content and conclusions of the report are the sole responsibility of the Auditor General.

"In addition, the audit team carried out extensive engagement with ten local communities to talk to ferry users about their experiences and views of ferry services."

Western Ferries boss Mr Ross said: "The figures as detailed were provided by the managing director of CalMac in formal evidence to the transport committee, as such how would I have any insight or foresight as to CalMac's proposed vessel design and the associated capital costs?"