Talks to avert strike action between the RMT and the Caledonian MacBrayne ferry company have broken down, the union's leader Bob Crow said.
Union officials want staff pay and conditions to be protected
The union is angry over plans to put the state-owned company's ferry routes out to commercial tender.
The Scottish Executive said there was no other option under European Union competition rules.
The RMT wanted assurances that the pay and conditions of workers will not be affected by the process.
In December, the executive suffered a rare defeat in parliament over the plans.
The proposal would have allowed private operators to bid to take over CalMac's Clyde and Western Isles routes.
Ministers said negotiations with Brussels are ongoing.
Earlier this month, the union accused ministers of pressing ahead with the tender without proper consultation.
The Gourock to Dunoon route has been specifically held apart from the other routes because a private company - Western Ferries - already competes on the route.
Eight companies have expressed an interest in taking over the route.
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.