World-Wide operates a fleet of more than 10 million deadweight tonnes, including 28 very large crude carriers.

Shell is the marine arm of the Royal Dutch Shell oil group.

Shell produced its paper amid a storm of public opinion led by environmentalists enraged by two recent oil disasters - the Aegean Sea in Spain and the Braer on the Shetland Islands.

Televised scenes of oil-blackened sea birds and despoiled shorelines have led to calls to limit both the size and age of vessels, and to extend exclusion zones around sensitive marine environments.

Shell said many ship owners lacked funds to improve safety standards because of competition from the estimated 20 per cent of tankers which were sub-standard and operated through ''less scrupulous charterers''.

Full enforcement would force the sub-standard vessels out of business and lift freight rates above the current level of US$10,000 to $25,000 a day for a large tanker, Shell said.

The group urged governments to improve the accountability of flag-state administrations in performing their responsibilities to the International Maritime Organisation (IMO) and to tighten links between ship ownership and country of registry.

This would curtail the freedom of shipowners to ''shop around between registries; until that is achieved the authority of flag states will continue to be ineffective''.

The group dismissed suggestions that limiting tanker size was a solution, claiming that this would increase the number of spills since they related to the number rather than the size of ships operating.

Mr Sohmen was also keen to urge a clampdown on sub-standard vessels.

''While the media and the politicians castigate the ship owners, and are quick to introduce new requirements or new prohibitions, governments are often quite slow in ratifying or enforcing already agreed international conventions aimed at forcing sub-standard vessels and operators out of business,'' he said.

''Given the international nature of the tanker industry, it is important that global regimes are applied consistently and universally, not local or regional rules that do not recognise the total commercial and operating backdrop.'' Shell recommended tighter manning, training and certification requirements, the targeting of port state inspections towards old ships, flags with poor records and non-members of the International Association of Classification Societies.

Shell said the frequency and scope of port state inspections should be increased and the sanctions against deficient ships increased - they were currently often told to put correct infringements ''before they return'', which could be years, if ever.

Mr Sohmen said it was important for the public to understand that reducing the threat of pollution ultimately would lead to higher transport and energy charges or a reduced number of operators and an even older world fleet.

The Shell report estimated that more than half of the existing tanker fleet and over a third of the very large crude carriers were more than 15 years old.

Mr Sohmen said: ''We cannot, of course, eliminate all accidents - no industry can do that.'' On this point, at least, the environmental group Greenpeace agrees.

Ms Tine Heyse, a campaigner at the group's Belgian office, said the amount of world oil traffic meant oil transport would always pose a danger to the environment.

''There are so many ships that sooner or later you will have an accident, so the only long-term solution is to reduce our dependence on oil,'' she said.

Greenpeace has issued a report - Energy without Oil - in which it advocates a 50 per cent decrease in world oil consumption within the next 40 years and has lobbied the European Parliament for an oil reduction policy.

In the meantime, Greenpeace wants an extension of exclusion zones and the early introduction of double hulls, even though, as Shell points out, double hulls would not have prevented the Aegean Sea or Braer disasters.

Locally, Friends of the Earth chairwoman Mary Riley said her group, which accepts funding from Shell for community environmental projects, believed the blueprint had not addressed all relevant issues.

''It still leaves the question of whether super tankers are appropriate to transport oil around the world . . . an accident involving a super tanker is catastrophic and it is very difficult for the environment to recover.''