Belgian group Jan De Nul is demanding repayment of all instalments paid to Croatia's troubled Uljanik Shipyard after it cancelled a dredger there.
It also wants interest on top, but no final amount has been revealed.
Government collateral had been pledged for some of the sum, the shipbuilder said on Tuesday.
Uljanik had been told last week that Jan De Nul would seek repayment.
"After months of negotiations between the client, Croatian state institutions and Uljanik, which were aimed at having the vessel completed at Uljanik instead of repaying the advance payments or transferring the vessel for completion to a foreign shipyard of the client's choice, the client has made a final decision to seek the repayment of all advance payments made, plus interest," a statement to the Zagreb stock exchange said.
The companies will continue to work on finding a solution to reduce the harm the latest developments are expected to cause to the state and Uljanik, it added.
The deal for the 26,600-dwt cutter-suction dredger was scrapped in January.
It was due in March.
The contract for hull 500 was cancelled due to Uljanik's inability to deliver the vessel in accordance with the conditions of the contract, the yard has previously said.
Uljanik has suffered a number of cancellations as it seeks to refinance in the face of creditor pressure and worker strikes.
