Workers at the Scottish state-owned ferry operator CalMac began balloting for industrial action on Wednesday of last week.
They are fighting threats to jobs, pensions and working conditions.
The RMT union members have received no assurances on jobs or conditions from bosses ahead of a possible takeover by a private operator.
They are also being asked to pay more, work longer and get lower pensions.
The Scottish National Party (SNP) government is tendering for bids to run ferry services from October next year.
Workers fear a £1 billion contract could be handed to outsourcing giant Serco.
It won the NorthLink ferry contract in 2012. This was despite NorthLink submitting a cheaper bid.
The SNP’s response to its handling of the NorthLink contract has been to withhold information and cite “commercial confidentiality”.