The same questions were raised when the Liberian-registered Braer foundered on rocks in the Shetland islands in January after losing power.
The International Transport Workers' Federation classes 16 countries as flags of convenience, including Liberia, Panama, the Bahamas, Bermuda and Cyprus, but there is a question mark over the exact definition.
The ITF suggests that it is where the true ownership and control of a vessel 'is found to lie elsewhere than in the country of the flag the vessel is flying'.
The British Trent is registered in Bermuda with British officers and a crew from Sierra Leone, while the Western Winner is registered in Panama with Korean officers and Burmese ratings. Companies 'flag out' ships to avoid strict controls and taxes imposed by the richer nations to maintain standards while countries which register the shipping see it as a good means of gaining foreign revenue. The British fleet has fallen from more than 1,300 vessels in 1979 to 290.
Shipping unions point to the lack of control by the countries with flags of convenience. The Bahamas, according to the marine officers' union Numast, has only 15 full-time surveyors for 973 ships and Cyprus 9 for more than 1,350 ships. But the International Maritime Organisation defends the practice. A spokesman said: 'It is impossible to define what is a flag of convenience and not all these flags have poor records. The issue is not the flag of ships but bad ships, and all countries have bad ships.'
BP, which flagged out its ships in 1986, defended its safety record. Dr Les Atkinson, chairman of BP Shipping, said: 'This ship, like all our ships, we believe was extremely well maintained to the highest international standards.' He had every confidence in the ship's crew, who were all of a high standard.
Although British Trent was Bermudan registered it met the same standards as those fixed by the British government, he said. 'The last accident of this kind which affected BP Shipping was in 1966. Until today, our recent safety record has been excellent.
'Officers and crew receive training that exceeds international standards. Our manning costs are 50 per cent above the industry average.'
John Prescott, Labour's transport spokesman, said: 'There must be an immediate ban on flags of convenience carrying hazardous cargo in British waters.'