Unions claim they feel like "Alice in Wonderland" in trying to argue the case against putting CalMac's west coast ferry services out to tender.
Unions sought the backing of MSPs over the CalMac tender process
Tom Kennedy, of the Transport and Salaried Staffs Association, made the claim while giving evidence to MSPs.
He said the government had got things wrong and there was no EU requirement to put services out to tender.
He and four other trade union witnesses claimed the tendering process could hit jobs and services.
The union representatives appeared before the Local Government and Transport Committee on Tuesday, where they won a sympathetic hearing from its members.
Mr Kennedy told them if there was a requirement to put CalMac's lifeline island ferry services up for grabs then the previous Tory government had failed to seek an exemption.
He said: "In that sense, we are in rather an Alice in Wonderland situation when we are trying to argue against regulations which should never have been applied in the first place.
"The approach adopted by the Scottish Executive has been to start off on the wrong foot, to look entirely in the wrong areas and not to consider that these are lifeline services."
The unions argued there was nothing in the regulations that required the executive to put the services out to tender.
They said any savings could only be made at the expense of the terms and conditions of ferry jobs and that if a new operator decided to base itself offshore, there could be big losses to the Treasury in national insurance contributions and taxes.
T&G shop steward John Docherty told the committee he knew of one village of 100 people, of which at least 10 were CalMac employees.
He said: "If there were 10 job losses, through bringing in foreign seamen, or whatever, that would have a huge impact on that community."
Mr Kennedy added: "Our members would not thank us for scaremongering - they expect us to be down-the-middle with them and give a realistic assessment of what's happening.
"And when my shore-based members look me in the eye and ask what will happen to their jobs with the tendering process, I'm afraid that the best answer I can give them is 'Wait and see - but you have to be extremely concerned about your future'."
The Scottish TUC, one of the union organisations giving evidence, said a report by Glasgow University had raised several concerns, including value for money for taxpayers, a threat to jobs, conditions and pensions of CalMac employees and the risk of poorer services, if the "unnecessary" tendering exercise went ahead.
The Rail Maritime and Transport (RMT) union has already announced it plans to ballot its members on industrial action.
The Scottish Parliament rejected plans for competitive tendering in December but the executive says European rules require it to allow private firms to bid against state-owned CalMac for operating the routes.