The 26 crew members of a tanker seized by Nigeria are being held without justification and should be freed immediately, says the International Chamber of Shipping (ICS).

The crew of mainly Indian and Sri Lankan seafarers on the 300,000-dwt Heroic Idun (built 2020) have been held under guard since 12 August by Equatorial Guinea and Nigeria over alleged oil theft and filing a false piracy report.

The ICS, which represents 80% of the world’s merchant fleet, called on the Nigerian authorities to review the case before a trial in Port Harcourt starts on 10 January. The men are being held on bail on board the ship.

Secretary general Guy Platten said: “This unacceptable detention of the Heroic Idun crew must be resolved as quickly as possible. It is essential that this situation is de-escalated, and that the seafarers and ship are released.”

The charges stem from a decision by the ship’s master to leave the Akpo oilfield off Port Harcourt when a ship approached in the night that could not be identified. The crew feared it was a potential piracy attack.

The vessel was subsequently discovered to be a Nigerian Navy vessel that was investigating if the VLCC was illegally lifting oil from an offshore terminal.

The Ray Car Carriers-owned Heroic Idun was sub-chartered by BP. The oil major and the ship’s owner, manager and insurer have all said that the Heroic Idun was cleared to load and the crew had done nothing wrong.

The case could undermine improvements by Nigeria and its neighbours in combating piracy in the region and its government’s commitments to seafarer welfare, said John Stawpert, a senior ICS official.

“Both the physical and mental wellbeing of the crew has suffered significantly as a consequence of the ordeal,” he said.

“Failure to pragmatically end this situation now will see it drag on into the next year, with no benefit to anyone, and so we urge its swift resolution.”

Several of the crew have suffered with typhoid and malaria.