BP's "environmental impact assessment" makes sobering reading. According to the company's own analysis, the well it proposes to drill in the sea off the Shetland Islands has the potential to unleash the world's biggest ever oil spill.

It is the comparisons with last year's Deepwater Horizon disaster that throw BP's worst-case scenario for North Uist into the sharpest relief. The damaged wellhead at the Macondo prospect disgorged 5 million barrels-worth of oil, polluting tens of thousands of square miles of the Gulf of Mexico, damaging more than 300 miles of the Louisiana coastline, and devastating local fishing and tourism industries.

Hampered by the depth of the water, engineers struggled for nearly three months before they succeeded in shutting it down. But a similar problem at North Uist – where the water is just as deep and often stormier – could make Deepwater Horizon look easy.

According to BP's estimates, the Scottish deep-water well could leak oil at a rate of 75,000 barrels per day, faster than Macondo at its height. And since any trouble could also take longer to fix – as much as 140 days, compared with "only" 88 for Deepwater Horizon – BP is modelling for a spill of more than 10 million barrels.

Given that the nearby Shetland Islands are the richest breeding ground for seabirds in Britain, and that the waters around them are home to substantial fish stocks and more than 20 species of whales and dolphins, the environmental impact of an oil slick in the area would be catastrophic.

There is one positive aspect to BP's nightmare vision. It is heartening that the company is setting out detailed disaster planning and attempting to put into practice the lessons learned from last year's tragedy in the Gulf of Mexico. That its analysis is largely in the public domain is also to be welcomed.

But even the reassurance that the worst-case scenario represents only the direst of all possible outcomes cannot lay concerns about North Uist entirely to rest. It is commendable that BP is not blind to the hazards. But the company's dramatic assessment of the scale of the risk cannot fail to alarm.