Ferry workers and supporters will renew calls for west coast services to remain in public ownership as MSPs prepare to discuss their future.
Members of the RMT union will lobby the Scottish Parliament ahead of a Labour debate on the tender for the Clyde and Hebrides ferry services.
CalMac's contract comes to an end next year and the Scottish Government has put it out to tender in compliance with EU law.
Labour's motion calls on ministers to halt the tender process and pursue an exemption for Scottish ferry services.
It states: "The Scottish Government could have pursued legal arguments with the European Commission to exempt lifeline Scottish ferry services from regulations requiring regular tendering of public-sector contracts, a process which is disruptive, expensive and biased toward private-sector bidders."
RMT said there was "massive public opposition" to privatisation of the services and called for the tendering process to be completed before next May's Holyrood election.
Fears over jobs, terms and conditions and pensions led to strikes and the suspension of two-thirds of services in June before a settlement was reached the following month.
The union is currently locked in a separate dispute with Serco over its operation of Caledonian Sleeper trains.
General secretary Mick Cash said: "The fight is now on to save CalMac's lifeline Clyde and Hebrides services from being snatched away by Serco - the specialists in failure.
"The idea that this company are fit to run our public services is ridiculous and the truth is that they shouldn't even be in the frame for these ferry routes.
"RMT continues to press for the tendering timetable to be sharpened up to allow for an announcement before the elections next year so that the public can judge the merits of any decision for themselves.
"There is no excuse for delaying the announcement which has huge implications for the communities which rely on these ferry services."
A spokesman for transport and islands minister Derek Mackay said: "Labour's attempts to stoke up grievance with false claims that services are being privatised is irresponsible and hypocritical given that they went through exactly the same tender process when they were in government - Labour's claims are also untrue.
"The Scottish Government is 100% committed to supporting these services in public ownership and this debate provides the opportunity to underline our commitment to these lifeline ferry services and the fair, open and transparent tendering process that is in place."
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