Union leaders have threatened strike action after accusing ministers of pressing ahead with plans to tender ferry services without consultation.
The decision to tender ferry services has proved controversial
Adverts are inviting bids for the Caledonian MacBrayne service from Gourock to Dunoon, despite MSPs voting against the process last year.
The Rail, Maritime and Transport (RMT) union said the move "came out of the blue".
However, the Scottish Executive said it revealed its intention in December.
The route has been specifically held apart from the other routes because a private company - Western Ferries - already competes on the route.
RMT national secretary Steve Todd said he was surprised by the tendering move and did not realise the plan was so advanced.
Speaking on BBC Radio's Good Morning Scotland programme, he said: "As far as we are concerned as a union, we have not been properly consulted about the effects of this tendering process.
"We are still saying there is no evidence which demonstrates that the executive has been pressurised into doing this."
The RMT is concerned about how a new operator may treat workers and perhaps employ them on lesser terms, he said.
Mr Todd added: "There are no assurances about what will happen to our members if another ferry operator comes in.
"If we fail to stop this, we want it firmly enshrined in our members' terms and conditions that everything they have will be firmly protected.
"We will ballot the whole Caledonian MacBrayne workforce (on strike action) because it doesn't just affect the Gourock-Dunoon service, it could have a knock-on effect for the rest of the employees in the company.
"So we will ballot our members with a view to taking industrial action."
Mr Todd said the union would probably know the result of any strike ballot within a month.
The Scottish Socialist Party said it would support the union on Friday and accused the Scottish Executive of acting like an "elected dictatorship" over CalMac.
The party's industrial organiser Richie Venton said: "The Scottish Parliament voted against the putting-out to tender of CalMac routes and yet the executive appear to care not one jot for the democratic process.
Jack McConnell has insisted ministers will not breach EU laws
"The SSP will stand shoulder to shoulder with the RMT in defence of their members' rights."
However, Labour's Western Isles MSP Alasdair Morrison criticised RMT leader Bob Crow for calling the strike ballot.
He said: "I know Bob Crow of old, I know he is a highly irresponsible union leader. It doesn't surprise me one bit that he has already moved, calling for strike action.
"He knows little about Caledonian MacBrayne and I can assure he cares less about the communities served by CalMac."
The press adverts invite commercial tenders but no government subsidy is included within the offer.
They also explain that there will be no timetabling restrictions but minimum standards will be set.
Western Ferries said it expects to bid for the route and the executive confirmed the route went out to tender from Friday.
A spokeswoman said: "We signalled our intention to put the Gourock to Dunoon route out to tender on 8 December and (Transport Minister) Nicol Stephen announced this to parliament at that time.
"This relates to EU rules on state aid and follows consultation with local people and has the backing of Argyll and Bute Council."
CalMac is a publicly-funded company which is wholly-owned by the executive but the government claims European rules mean that all its west coast routes must be opened up for privatisation.
The CalMac crew unions lobbied parliament on Thursday demanding the executive abandon plans to put the services out to tender.
Supported by the Scottish Trade Union Congress (STUC), they claim any tendering will lead to less efficient services, bring additional costs to the taxpayer and have a detrimental effect on the island communities off the west coast of Scotland.
But on Friday, Transport Minister Nicol Stephen claimed other CalMac services would not be put out to tender until further discussions with the European Commission had taken place.
Nicol Stephen has promised to discuss the tender issue with the EC
He said: "We're not going to tender on the Gourock - Dunoon services or any of the other CalMac services.
"What we're going to do is approach the European Commission, we're going to clarify the issue, in terms of EU law and the requirements to tender, and then we're going to report back to parliament and that's my absolute guarantee."
First Minister Jack McConnell has previously insisted ministers would not break the EU law by preventing the process from going ahead.
In December, the executive suffered only its second defeat on a parliamentary vote when MSPs defeated a motion to endorse the move by a single vote.
Since then, Mr Stephen has been in talks with the European Commission, which enforces the tendering process, in an effort to broker a deal on the issue.