BP is making contingency plans to fight the largest oil spill in history, as it prepares to drill more than 4,000 feet down in the Atlantic in wildlife-rich British waters off the Shetland Islands.

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Internal company documents seen by The Independent show that the worst-case scenario for a spill from its North Uist exploratory well, to be sunk next year, would involve a leak of 75,000 barrels a day for 140 days – a total of 10.5 million barrels of oil, comfortably the world's biggest pollution disaster.

This would be more than double the amount of oil spilled from its Deepwater Horizon well in the Gulf of Mexico last year, which had a maximum leak rate of 62,000 barrels a day in an incident lasting 88 days – and triggered a social, economic and environmental catastrophe in the US which brought the giant multinational to the brink of collapse.

The North Uist well, in a seabed block named after the Hebridean island but located 80 miles north-west of Shetland, is part of BP's ongoing attempts to open up the West of Shetland sea area, sometimes referred to as the "Atlantic Frontier", as a rich new oil province to replace the dwindling productivity of the North Sea.

The project appeared to have been shelved by the former BP chief executive Tony Hayward last year in the aftermath of Deepwater Horizon and the barrage of criticism directed at the company for its safety record. But it is now going ahead, and the well will be drilled by a drilling ship, the Stena Caron, some time from January onwards, as long as it is given a licence by the Energy Secretary, Chris Huhne.

The company already has three West of Shetland wells producing oil, at depths from 140 to 500 metres (460 to 1,640ft). But North Uist, described by BP as "stepping out, in terms of depth", will be nearly three times as deep, at 1,290m below the surface, in immensely testing conditions similar to those of its ill-fated Gulf well, which was located 1,500 metres down, and began its unprecedented "gusher" leak in April last year.

The difficulty of capping a gushing well at such depths, vividly illustrated by the three months it took for Deepwater Horizon to be staunched, is greatly concerning British environmentalists who point out that the waters which might be affected by a North Uist spill are among the most wildlife-rich in all the UK.

Seabirds including many rare species are found in enormous concentrations on Shetland, the nearest landmass to any spill, and in the surrounding waters, which also contain large numbers of whales, dolphins and seals, as well as substantial fish stocks.

A major destination for wildlife tourism, Shetland has already been badly affected by a previous oil spill, that of the tanker MV Braer, which ran aground on Shetland in January 1993. BP documents referring to the North Uist project themselves list more than 20 vulnerable Shetland nature sites, including eight Special Protection Areas, two Special Conservation Areas and 12 Sites of Special Scientific Interest, which involve the breeding grounds of otters and rare birds such as the great skua, the red-throated diver and Leach's petrel.

"This project is so risky that even BP is quietly planning for the possibility of the world's worst ever oil spill happening off Scotland's precious coastline," said John Sauven, executive director of Greenpeace UK.

"It would be utterly reckless for Chris Huhne to approve this plan as if the Deepwater Horizon disaster never happened.

"Instead of chasing the last drops of oil from one of our country's most sensitive and important natural environments, ministers should be developing a comprehensive plan to get us off the oil hook."

A spokesman for BP said that the company was legally obliged to model the worst-case scenario, "but the reality is, the chances of a spill are very unlikely". Since Deepwater Horizon, he added, BP had invested "a huge amount of time and resources strengthening procedures, investing in additional safety equipment and further improving our oil spill response capability".

In particular, a major new well-capping device, designed for use at depths of up to 10,000ft, has been constructed, tested and made available, and could quickly be deployed, and any leak from North Uist is likely to be at a much lower pressure than that in the Gulf.

"We are confident that the improvements that have been made provide the level of assurance necessary against the risks," the BP spokesman said.

North Uist: The story so far

In the storm of criticism of its safety record that followed the Deepwater Horizon blow-out, BP blew hot and cold about drilling the North Uist well. After confirming that it would go ahead, in August 2010, the company faced more criticism that such a similar deep well was inappropriate in the aftermath of the disaster in the Gulf of Mexico. Tony Hayward, BP's chief executive at the time, hinted to the House of Commons Energy Select Committee in September 2010 that BP would hold its plans for deep water drilling off the Shetlands. He left the company shortly afterwards, and a final decision was taken to go ahead with North Uist, although more than a year later than originally intended.