Work has started to close down four platforms in the Brent field, which started producing oil about 115 miles east of Shetland 40 years ago but is “no longer economically viable”.
The company plans to submit a “comprehensive decommissioning plan” for the Brent field to the Department of Energy and Climate Change (DECC) before the end of 2016 but will seek an exemption from the UK Government to leave the legs on safety grounds.
Each concrete and steel leg is said to weigh about 300,000 tonnes.
But WWF Scotland believes they could pose an environmental risk if left in the North Sea.
A Shell spokesman said: “Following extensive consultation with a wide range of stakeholders, Shell intends to submit a comprehensive decommissioning plan for the Brent oil and gas field to DECC before the end of 2016.
“As part of this plan, Shell will recommend that the gravity base structures of three of the Brent platforms should be left in place. Work is continuing to prepare the Brent Delta platform ahead of its topsides being removed in a single lift operation, which is expected to take place in 2017.”
WWF Scotland director Lang Banks said: “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.