Oil companies must honour international obligations, by removing huge and potentially hazardous subsea foundations and supports after they decommission their North Sea platforms, conservationists insist.

WWF was reacting to the news that Royal Dutch Shell wants to leave behind steel and concrete structures as large as the Empire State Building when it abandons its Brent field, 115 miles north-east of the Shetland Islands, one of the biggest oil and gas fields in the North Sea.

In the 1990s, Brent met 13 per cent of the UK's oil and gas needs, and has raised around £20bn of revenues for the Treasury since it started producing in the 1970s.

But three of the four Brent platforms Alpha, Bravo and Delta, have already ceased production and Charlie is expected to stop within the next few years.

Its decommissioning is being seen as an important test of the international regulations which demand all traces of oil and gas production are removed at the end of offshore operations.

The topsides of the oil and gas rigs, will be removed and dismantled onshore. But there is particular concern over the likes of the "cells", which surround the base of the main subsea structures and contain sediment, water and oil.

Shell argues that the safety and environmental risks removing such infrastructure would far outweigh the benefits. Its plans will go to the UK's Department for Energy and Climate Change by the end of this year, when there will be a public consultation.

WWF Scotland director Lang Banks said:

“While removing these structures is not without environmental risk, neither is leaving them lying on the seabed to slowly breakdown over hundreds of years. Given the potential impacts on the marine environment and other sea users, we will be carefully examining the final proposals that go out for consultation.

“The industry pushed the boundaries of science and engineering to access North Sea oil and gas. Having made massive profits over the last few decades, it’s only right that it should push those limits once again to clean up their potentially hazardous legacy and protect the marine environment."

He said given the enormous size of the rigs and the iconic nature of the Brent field, its decommissioning was being watched closely. So the highest possible benchmarks should be set for the rest of the industry to follow.

"If done right, then it could open the door for Scotland to lead a new multi-billion pound, global decommissioning industry. "If politicians are prepared to grasp the nettle, then the decommissioning of the Brent field has the opportunity to mark a new and positive chapter in the history of oil and gas in the UK,” he said.

Shell said it had consulted widely, including with environmental groups and the fishing industry,