Shipowners will abandon the Gulf of Aden and large parts of the Indian Ocean if a US threat to ban ransom payments becomes law, a seminar in Brussels heard this week.

Owners would be obliged to avoid the Suez Canal and detour south via Madagascar if they were denied the option of paying pirates to release crews, the seminar heard.

Speakers said they feared talk among US law-makers of outlawing ransoms would become official policy and would be submitted to the United Nations for a resolution.

"Consultations with shipowners have made it very clear that if you are denied the right to pay ransoms for release of crews you will take into consideration the wisdom of continuing to trade in that area," said Peter Swift, managing director of tanker association Intertanko. Crew currently held hostage would be abandoned, while other crews would be unwilling to ply trades "not just in the Gulf of Aden but the Indian Ocean too", he said. There would be "very significant disruption" in trade to Europe.

Giles Noakes, head of the security department at maritime body BIMCO, said: "Criminalisation of the victims of piracy is immoral."

"If the US succeeds [the Gulf of Aden] will become a no-go area," said Jon Whitlow of the International Transport Workers' Federation. "Even the US maritime unions are vehemently against this."

Lloyd's Market Association senior executive for underwriting Neil Roberts said a law banning ransoms would result in an immediate loss of around $500m given the value of the 10 vessels and cargo held on average at any one time in Somalia.

A Brussels lawmaker told Lloyd's List of concerns that such a ban could weaken Egypt's government. "If ships avoid the Gulf of Aden, think what that will mean for Suez Canal revenues - and think what that in turn will mean for Egypt, one of the countries which is key to stabilising the whole region," he said.