The invitation to tender has been released for an estimated £370m contract to operate the Northern Isles Ferry Services, which is designed to adapt to changes in demand.
Three bidders are reported to be involved in talks with Transport Scotland.
Serco, which runs NorthLink Ferries, and CalMac, which is owned by the Scottish government, are thought to be in the running. The name of the third company has not been released.
The decision to put the contract out to tender rather than deliver it in-house was announced last year. Scottish ministers said it had become clear that the preference of the local communities was for the contract to go out to market.
The winning company will operate ferry services to or from harbours in Aberdeen, Kirkwall, Lerwick, Scrabster and Stromness.
The existing public services contract expires on 31 October this year and the new deal will run for a maximum of eight years; an initial six with the chance for a two-year extension.
Minister for energy, connectivity and the islands, Paul Wheelhouse, said: 'A key benefit of this new contract is the in-built flexibility, allowing the ferry services to be amended to reflect future changes in demand across the important sectors of the islands’ economies – tourism, fishing, food and drink, aquaculture and farming.
'Scottish ministers will also retain control of all of important issues, such as fares and timetables, through the public service contract.'
The contract value may increase should additional tonnage and/or additional routes be introduced during the contract period in order to meet future service needs.