Shell is applying to leave the 300,000-tonne concrete bases beneath three of the platforms as it decommissions its Brent oilfield, while their massive concrete legs would be left sticking out of the sea.

But the Dutch oil giant is planning to remove the topsides of all four Brent facilities (Alpha, Bravo, Charlie and Delta) according to the plans lodged today with the UK's Department for Business, Energy and Industrial Strategy (BEIS). A 60 day public consultation has now been triggered.

However the OSPAR Convention, governing marine pollution and dumping waste at sea, states oil companies must remove every part of their platforms when they are shut down. Any relaxation of this requirement is likely to be met with stiff opposition from environmental groups and others who see Brent as a crucial test of the convention The OSPAR rules came into force in 1998, after a battle between Shell and ecological activists over the disposal of the Brent Spar storage buoy. But they do allow operators to apply for exemption if the risks in removing parts of their structures are too great.

Shell said recognises its plans would "require consultation with and support of the OSPAR Commission."

The Brent field, located 115 miles north-east of the Shetland Islands has produced around three billion barrels of oil since production commenced in 1976, which is almost 10 per cent of UK production.

Duncan Manning, Shell's Brent Decommissioning Asset Manager, said: “Shell has undertaken thorough analysis, extensive scientific research and detailed consultation with over 180 stakeholder organisations over the past 10 years. Working within the tightly defined regulatory process, we believe that our recommendations are safe, technically achievable, environmentally sound and financially responsible. Shell encourages all those with an interest in the decommissioning of the Brent field to review, reflect on and respond to this consultation document.”

He said work to prepare for Brent decommissioning started in 2006. More than 300 expert studies had been completed and the results analysed and verified by a group of independent scientists.

Conservationists are watching the Brent story closely. WWF Scotland director, Lang Banks, said: “With almost 4,000 pages of technical documents to plough through it will take some time to understand the full potential impact of Shell’s plans on the marine environment."

However having been involved in the stakeholder engagement process for several years, he already knew Shell would need to be pressed to do more to deal with its potentially hazardous legacy.

He said "Oil and gas companies operating in the North Sea have a legal, as well as moral, obligation to clean-up their mess. Having once pushed the boundaries of science and engineering to secure the oil and gas beneath the seabed, the industry should show the same innovation when it comes to decommissioning.

"The OSPAR agreement is there to make sure the marine environment is protected and should be followed. The rules already allow for companies to seek permission to leave some material behind - such as the massive concrete legs - where moving it would pose an unacceptable risk to staff or the environment. We accept this principle.

"If the oil within the cells and some of the drill cuttings occur above limits that internationally agreed standards recommend, then they should be removed in order to prevent damage to the environment. The main thing preventing this from being done in this particular case is the cost. Shell should do the right thing and remove these potentially polluting materials.”

He recognised the economic opportunities arising from decommissioning and given the enormous size of the rigs and the iconic nature of the Brent field, its progress was being studied around the globe.

"It (Shell) should therefore be aiming to set the highest possible benchmarks for the rest of the industry to follow. If done right, it could open the door for this country to lead a new multi-billion pound, global decommissioning industry that could create thousands of jobs as we continue our transition away from fossil fuels."