Illegal fishing fleets are plundering the seas by taking advantage of rules allowing them to adopt "flags of convenience", a report has claimed.
The report by the UN International Transport Workers' Federation and WWF calls for the abolition of the system, which it describes as corrupt.
It says some vessels also flout health and safety rules and use forced labour.
The annual value of illegal fishing has been estimated at $1.2bn (£679m), but the real figure could be far more.
Flags of convenience can be bought, sometimes over the internet, for just a few hundred dollars. This can give a ship the appearance of legitimacy within hours.
But the marine registry of one country said some of the information contained in the report was out of date.
The country under whose flag a boat sails is responsible in international law for controlling the activities of that vessel.
This includes ensuring that it abides by national and international regulations, such as fishing quotas and labour and safety standards.
However, the report says that some countries allowing boats to fly their flags for a fee fail to enforce such rules.
"We know of fishing vessels that carry up to 12 different flags on board, and they re-flag their ship at sea," Dr Claude Martin, director-general of WWF, told the BBC.
"If landlocked countries sell flags of convenience, they couldn't care less what's going on at sea.
"It is one of the most unregulated, uncontrolled businesses that is going on."
Some of the most popular countries for crews seeking flags of convenience are Belize, Honduras, Panama; and St Vincent and the Grenadines.
The International Merchant Marine Registry of Belize said data contained in the report on vessels registered with the country was out of date. In a statement, it added that consideration had not been given to actions taken by Belize to regulate its High Seas Fishing Fleet.
These include deregistering 513 fishing vessels during the period from September 2001 to 11 October 2002. In addition, the registry said Belize has implemented vessel registry and catch reporting legislation and signed up to the UN Fish Stocks Agreement.
Shipping countries in the European Union have the largest number of ships flying flags of convenience.
Spain, which receives the most generous EU fishing subsidies, tops the list with 46.
Many vessels, particularly those pursuing high value fish such as swordfish and tuna, transfer their catch to other boats to "launder" their illegally caught fish.
In addition to threatening the world's fisheries, bycatch - the incidental capture of non-targeted species - from pirate fishing operations is a serious threat to sea turtles, albatross, sharks and a range of other species.