The RMT union wants Scottish ferry company NorthLink to be brought under state control.
It is holding a public meeting on Shetland to discuss the issue on Friday.
The call has been prompted by the Scottish government saying it will shortly decide whether they will put the next contract for Northern Isles Ferry Services (NIFS) out to tender or not.
RMT claimed contract holder Serco had "overseen a decline in performance, passenger numbers and freight movements whilst receiving a significantly higher subsidy compared to the previous operator."
A Serco spokesman told the Shetland News the ferry operation was "a valued partner of the Scottish Government and the communities that it services and we are very proud of the lifeline service that we provide to the people and businesses of the Shetland and Orkney isles.
"Since Serco took on the service in 2012 we have carried a total of 1.5m passengers, an 8% increase since the start of the contract, as well as 20% more vehicles.
"We have made significant improvements to both the vessels and the on-board offering and 99% of our passengers rated their overall experience as positive, which we are delighted about."
