The 974-ft (297m) tanker, which was carrying 100,000 tons of crude oil, hit Pollard's Rock in the Seven Stones reef.

The oil patch already forming is believed to be the biggest ever to threaten the West Country coastline.

There are fears that the beaches of Cornwall, Devon and Dorset could be hit by the slick.

Cornish beaches

The Navy is plotting the direction of the oil, which is heading slowly towards the French coast, but say a slight change in wind direction could send it towards Cornish beaches.

Troops will regularly patrol around the coastline to give some indication of where the oil might be heading.

The 2nd Infantry Brigade, stationed at Plymouth, is standing by to assist if the oil threatens beaches.

A Penzance lifeboat official said the position was "serious" because the oil could cover the whole of the south-west coast for the next year.

The crew of the Seven Stones lightship, two miles off the reef, said they realised the tanker was in danger when she was still a mile from the disaster.

There are also fears that the supertanker could catch fire or break-up in heavy seas.

Up to 10,000 gallons of oil detergent are on their way from Grangemouth in Scotland to Falmouth.

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