The three companies tendering for the next Northern Isles ferry contract have been revealed by the Scottish government.
However, the procurement is facing a legal challenge from Orkney-based commercial operator Pentland Ferries.
The contract for routes between mainland Scotland and Orkney and Shetland - worth an estimated £370m - is expected to be awarded in the summer.
It will run for eight years.
Pentland Ferries has lodged a petition for judicial review of Scottish Ministers' decision to subsidise the provision of public services in relation to Northern Isles ferry services.
The company has also lodged a related complaint with the European Commission.
Minister for the Islands, Paul Wheelhouse, pledged that the procurement process - launched in September - would continue as planned, pending the outcome of the judicial review and the complaint to the EC.
The three potential bidders who have been invited to tender for the contact are CalMac Ferries Ltd, Förde Reederei Seetouristik GmbH & Co. KG (FRS), and Serco NorthLink Ltd.
Serco NorthLink are the current operators.
The Scottish government owns Caledonian Marine Asset Ltd (CMAL) - the body that buys and leases ships for CalMac.
Final tenders are to be submitted by the end of April, and the contract is expected to be awarded in August.
Mr Wheelhouse said the contact had flexibility built in, to allow the services to meet demand across important sectors to the islands' economies, including tourism, fishing, food and drink, aquaculture and farming.
He added: "Scottish Ministers will retain control of all of important issues, such as fares, timetables and routes to be served, through the public service contract. The services are being tendered as a single bundle."
Mr Wheelhouse said of the judicial review that what he could say was limited because of the legal challenge.
However, he added: "We will robustly defend our position in relation to the continued provision of subsidy in support of these lifeline ferry services.
"The procurement process will continue as planned, pending the outcome of the judicial review and complaint to the European Commission."
In 2012, ministers awarded a six-year contract to Serco.