Three vast concrete blocks, each weighing as much as the Empire State Building, may be abandoned on the floor of the North Sea, as part of Shell's decommissioning process of its Brent oil platforms.

Royal Dutch Shell is expected to seek approval to leave the caissons at the base of its platforms, located north-east of the Shetland Islands, on the seabed - amounting to a sanctioned breach of international regulations.

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 and any relaxation of this requirement is likely to be met with stiff opposition from environmental groups.

The rules came into force in 1998, after a battle between Shell and ecological activists over the disposal of the Brent Spar storage buoy.

Initially the oil giant wanted to dump the buoy in the deep waters of the north Atlantic, but backed down after global pressure from governments and campaigners.

Shell is in the process of consulting with experts and stakeholders on the future of the three reinforced concrete caissons at Brent, which individually stand 160m tall and weigh 300,000 tonnes.

It said a “range of options for the Brent field” were being considered before a final proposal is submitted to the Department for Energy and Climate Change for approval.

“One of the options being considered for the concrete gravity based structures of three of the platforms is to leave them in place. Given these complex structures weigh over 300,000 tonnes each and were never designed to be re-floated, the engineering challenge is immense,” a Shell spokesperson said.

Despite the head of an independent review for Shell stating there was an environmental case for the concrete bases to remain in situ, environmental organisations remain opposed to the proposal.

Greenpeace’s chief scientist Doug Parr said: "Nothing has changed over the last two decades since Shell last got into trouble over trying to dump old kit from oil fields at sea. If rigs can be recycled we expect oil companies, like all companies in the EU are expected to do, to avoid contamination of the marine environment and maintain a drive towards a circular economy by re-using resources.

“If there are compelling environmental reasons for doing anything different, they can go through the existing international legal process. There is no rationale for rules to be relaxed," he added.

It would not be the first time an Ospar opt-out was granted, however. Both Total of France and ConocoPhilips, a US oil company, secured an exemption from the convention for their platforms.

But concerns are growing that oil companies are pushing for a permanent watering down of Ospar rules.

Shell, BP and ExxonMobil, among others, have funded a study into the effect of leaving man-made structures in the sea, due to be published in 2017.

The price of oil has been falling steadily to an 11 year low of $35 a barrel and oil companies are looking for more cost-effective ways to decommission former platforms.

Production from the Brent oilfield began in 1976 and has accounted for a tenth of all the UK North Sea oil and gas brought onshore in the intervening years.

In total the three oil platforms to be decommissioned are the same height as the Eiffel Tower, standing at 300m tall, including the topside of the rig, which contains the accommodation and service areas.

Shell plans to use the world's largest double-hulled ship to remove the topside section separately, which will then be transported to Hartlepool for recycling next year.